Constant current source circuit and signal processing circuit

Abstract

The present disclosure relates to a constant current source circuit and a signal processing circuit. The constant current source circuit comprises: a voltage input terminal (V_set); a current output terminal (I_out); a range resistor (40); a first amplification circuit (10), a non-inverting input terminal of a first operational amplifier (U1) being connected to the voltage input terminal (V_set) of the constant current source circuit and configured to receive an input voltage at the voltage input terminal (V_set); a second amplification circuit (20), an output terminal of a second operational amplifier (U2) being connected to an inverting input terminal of the first operational amplifier (U1); and an isolation and sampling circuit (30) being arranged between the inverting input terminal of the second operational amplifier (U2) and an output terminal of the range resistor (40) and configured to extract an output voltage obtained at the output terminal of the range resistor (40), wherein the second operational amplifier (U2) is configured to proportionally amplify the obtained output voltage.

Description

Technical Field

[0001] The embodiments disclosed herein generally relate to the field of signal processing, and in particular to constant current source circuits. Background Technology

[0002] With the iterative development of technology, fields such as semiconductor detection, photoelectric detection, biomedicine, and materials characterization have placed higher demands on signal processing precision. In some applications, constant current source circuits are required to convert voltage signals from sensors and / or digital-to-analog converters (DACs) into high-precision constant current outputs for further signal processing.

[0003] The range of the weak current that a constant current source circuit can provide is an important performance indicator. Several constant current source circuits for weak currents have been proposed; for example, CN102455727B discloses a range covering 100pA to 1μA. However, this range is still insufficient for more precise instruments. Further improvements are needed to provide constant current source circuits that meet the requirements of even weaker currents. Summary of the Invention

[0004] One of the purposes of this disclosure is to provide a solution or at least a mitigation of one or more of the aforementioned technical and / or other potential problems.

[0005] According to a first aspect of this disclosure, a constant current source circuit is provided. The constant current source circuit includes: a voltage input terminal; a current output terminal; a range resistor, the output terminal of which is connected to the current output terminal of the constant current source circuit; a first amplifier circuit configured to provide an output current based on an input voltage at the voltage input terminal, and including a first operational amplifier, the non-inverting input terminal of which is connected to the voltage input terminal of the constant current source circuit and configured to receive the input voltage at the voltage input terminal, wherein an integrating capacitor is disposed between the output terminal and the inverting input terminal of the first operational amplifier; a second amplifier circuit configured to proportionally amplify the output voltage at the current output terminal, and including a second operational amplifier, the output terminal of which is connected to the inverting input terminal of the first operational amplifier; and an isolation and sampling circuit disposed between the inverting input terminal of the second operational amplifier and the output terminal of the range resistor and configured to extract the output voltage acquired at the output terminal of the range resistor, wherein the second operational amplifier is configured to proportionally amplify the acquired output voltage.

[0006] In some embodiments, the isolation and sampling circuit includes a third operational amplifier disposed between the inverting input of the second operational amplifier and the output of the range resistor, wherein the input bias current of the third operational amplifier is less than the input bias current of the first operational amplifier and the second operational amplifier and / or the input impedance of the third operational amplifier is greater than the input impedance of the first operational amplifier and the second operational amplifier.

[0007] In some embodiments, the third operational amplifier is implemented as a voltage follower.

[0008] In some embodiments, the isolation and sampling circuit includes a third operational amplifier, which is disposed between the inverting input of the second operational amplifier and the output of the range resistor, and the third operational amplifier is implemented as an electrometer operational amplifier.

[0009] In some embodiments, the third operational amplifier has an input bias current on the order of fA to be able to extract at least a current on the order of 0.01pA to 1pA, for example, 0.1pA, flowing through the output of the range resistor.

[0010] In some embodiments, the non-inverting input of the third operational amplifier is connected to the output of the range resistor, the inverting input of the third operational amplifier is connected to the output of the third operational amplifier, and the output of the third operational amplifier is connected to the inverting input of the second operational amplifier.

[0011] In some embodiments, the constant current source circuit further includes an inverting voltage branch located at the inverting input terminal of the second operational amplifier. The inverting voltage branch includes a first resistor and a second resistor connected in series with each other. The other end of the first resistor is connected to the output terminal of the second operational amplifier, and the other end of the second resistor is connected to the output terminal of the third operational amplifier. The inverting input terminal of the second operational amplifier is connected to the connection node between the first resistor and the second resistor.

[0012] In some embodiments, the constant current source circuit further includes a non-inverting voltage branch located at the non-inverting input terminal of the second operational amplifier. The non-inverting voltage branch includes a third resistor and a fourth resistor connected in series with each other. The other end of the third resistor is connected to the output terminal of the first operational amplifier, and the other end of the fourth resistor is grounded. The non-inverting input terminal of the second operational amplifier is connected to the connection node between the third resistor and the fourth resistor.

[0013] In some embodiments, the first operational amplifier has a first input bias current, the second operational amplifier has a second input bias current, and the third operational amplifier has a third input bias current, wherein the third input bias current is less than either the first input bias current or the second input bias current.

[0014] In some embodiments, the first input bias current and / or the second input bias current are in the pA or nA range; the third input bias current is in the fA range; and / or the third input bias current is one of the following value ranges: not greater than 80 femtoamperes, not greater than 50 femtoamperes, not greater than 30 femtoamperes, not greater than 20 femtoamperes, and not greater than 10 femtoamperes.

[0015] In some embodiments, the first operational amplifier has a first input impedance, the second operational amplifier has a second input impedance, and the third operational amplifier has a third input impedance, wherein the third input impedance is greater than the first input impedance and the input impedance; and / or the third input impedance is not less than one of the following values: 1TΩ, 50TΩ, or 100TΩ.

[0016] In some embodiments, the range resistor includes a plurality of resistors having different resistance values ​​from each other; the constant current source circuit also includes a switch disposed between the range resistor and the output of the first operational amplifier, the switch being configured to selectively connect one of the plurality of resistors to the output circuit of the constant current source circuit.

[0017] In some embodiments, the plurality of resistors includes a first range resistor with a resistance value selected from one of the following ranges: 0.1 GΩ to 800 GΩ, 1 GΩ to 500 GΩ, 10 GΩ to 100 GΩ; and / or the plurality of resistors includes a second range resistor with a resistance value selected from one of the following ranges: 1 KΩ to 10 MΩ.

[0018] According to another aspect of this disclosure, a signal processing circuit is provided, including a constant current source circuit according to any one of the first aspects.

[0019] The constant current source circuit and signal processing circuit disclosed herein enable the extraction of weak currents on the pA level. In some embodiments, full-range compatibility is achieved; the three-level architecture can operate stably across the entire pA / nA / μA / mA range, simplifying the circuit structure. Attached Figure Description

[0020] The above and other objects, features, and advantages of embodiments of the present disclosure will become readily apparent from the following detailed description taken in conjunction with the accompanying drawings. Several embodiments of the present disclosure are illustrated in the drawings by way of example and not limitation.

[0021] Figure 1 A schematic diagram of the constant current source circuit according to an embodiment of the present disclosure is shown.

[0022] Figure 2 A schematic diagram of a constant current source circuit according to another embodiment of this disclosure is shown.

[0023] Throughout the accompanying drawings, identical or similar parts are indicated by the same and similar reference numerals. Detailed Implementation

[0024] Preferred embodiments of the present disclosure will now be described in more detail with reference to the accompanying drawings. While preferred embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that the present disclosure will be thorough and complete, and will fully convey the scope of the present disclosure to those skilled in the art.

[0025] The term "comprising" and its variations as used herein signify an open-ended inclusion, i.e., "including but not limited to". Unless otherwise stated, the term "or" means "and / or". The term "based on" means "at least partially based on". The terms "one example embodiment" and "one embodiment" mean "at least one example embodiment". The term "another embodiment" means "at least one additional embodiment". Terms such as "upper", "lower", "front", and "rear", indicating placement or positional relationship, are based on the orientation or positional relationship shown in the accompanying drawings and are used only for the purpose of describing the principles of this disclosure, and are not intended to indicate or imply that the elements referred to must have a specific orientation, be constructed or operated in a specific orientation, and therefore should not be construed as limiting this disclosure.

[0026] In the field of precision current source design, operational amplifier feedback constant current circuits are typically used to implement the current source. However, there are significant difficulties in implementation when the output current is very low (e.g., as low as 1pA).

[0027] First, there's the issue of operational amplifier bias current interference. In scenarios with very low current, the op-amp's own input bias current will overlap with the weak output current, causing significant errors. For example, the input bias current of a typical op-amp is in the pA to nA range. For a pA-level output current (such as 10pA), the bias current itself will cause an error of more than 10%, failing to meet the accuracy requirements in the pA range, such as ±1.4pA, ±1pA, ±0.5pA, ±0.1pA, or even higher.

[0028] Secondly, it is difficult to extract weak voltage signals: the voltage signal generated by the pA-level current on the ultra-high impedance feedback resistor (such as 100GΩ) is extremely weak (mV level), and traditional circuit structures cannot accurately extract this signal without introducing additional errors.

[0029] Furthermore, weak currents are highly susceptible to noise interference, which places higher demands on the design of gain circuits. For example, when high-frequency noise is present in the circuit, these high-frequency signals severely affect the stability of the output current.

[0030] According to this disclosure, a constant current source circuit and a signal processing circuit are provided to solve one or more of the above-mentioned technical problems. The constant current source circuit according to embodiments of this disclosure is described in detail below with reference to the accompanying drawings.

[0031] Figure 1 A schematic diagram of the constant current source circuit 1 according to the first embodiment of this disclosure is shown. Figure 1 As shown, the constant current source circuit 1 includes: a voltage input terminal V_set, a current output terminal I_out, and a range resistor 40. The voltage input terminal V_set can receive a voltage signal from upstream of the constant current source circuit 1. These voltage signals can, for example, come from a sensor and / or an ADC. The current output terminal I_out can be connected to the application load circuit.

[0032] The range resistor 40 is typically implemented with high impedance to reduce the impact of the constant current source circuit 1 on the applied load circuit. In some embodiments, the range resistor 40 may include multiple resistors connected in parallel with each other, each resistor having a different resistance value. Different current ranges are covered by setting different resistance values. The constant current source circuit also includes a switch S, which selectively switches the resistor with a predetermined resistance value into the constant current source circuit 1.

[0033] In some embodiments, the constant current source circuit 1 can cover a range from pA (e.g., 0.1pA, 0.01PA, or even lower) to mA (e.g., tens of mA, hundreds of mA, or above). As an example, for the pA range, resistors such as tens to hundreds of GΩ (e.g., 100 GΩ) can be set; for the nA range, resistors such as hundreds to several GΩ (e.g., 1 GΩ) can be set; and for the μA / mA range, resistors such as hundreds of Ω to hundreds of MΩ (e.g., 1 kΩ to 10 MΩ) can be set. It should be understood that the above resistor values ​​and the number of resistors set are merely exemplary. Appropriate numbers and / or appropriate resistor values ​​can be set as needed.

[0034] The constant current source circuit 1 also includes a first amplifier circuit 10, a second amplifier circuit 20, and an isolation and sampling circuit 30.

[0035] The first amplifier circuit 10 is configured to provide output current based on the input voltage at the voltage input terminal V_set. For example... Figure 1As shown, the first amplifier circuit 10 may include a first operational amplifier U1. The non-inverting input terminal of the first operational amplifier U1 is connected to the voltage input terminal V_set of the constant current source circuit and is configured to receive the input voltage at the voltage input terminal V_set. An integrating capacitor C is provided between the output terminal and the inverting input terminal of the first operational amplifier U1. By providing the integrating capacitor, high-frequency noise can be effectively suppressed, and the output voltage of the first amplifier circuit can be smoothed.

[0036] The second amplifier circuit 20 is configured to proportionally amplify the output voltage at the current output terminal I_out. For example... Figure 1 As shown, the second amplifier circuit 20 includes a second operational amplifier U2. The inverting input of the second operational amplifier U2 is connected to the output of the range resistor 40 via an isolation and sampling circuit 30 and is configured to proportionally amplify the output voltage obtained at the output of the range resistor 40. The output of the second operational amplifier U2 is connected to the inverting input of the first operational amplifier U1.

[0037] An isolation and sampling circuit 30 is disposed between the inverting input of the second operational amplifier U2 and the output of the range resistor 40 and is configured to extract the output voltage acquired at the output of the range resistor 40. In some embodiments, the isolation and sampling circuit 30 has a current resolution on the order of fA. This means that the isolation and sampling circuit 30 can effectively identify extremely weak currents (e.g., 0.1 pA, 0.01 PA, or even lower) flowing through the output of the range resistor 40.

[0038] According to this disclosure, the output of the second amplifier circuit 20 is connected back to the inverting input of the first amplifier circuit 10 to form a closed-loop negative feedback, achieving precise constant current control and high output current stability. Furthermore, the isolation and sampling circuit 30 effectively extracts the voltage at the output of the range resistor 40. The second amplifier circuit 20 proportionally amplifies the extracted voltage to adjust the gain, while the first amplifier circuit 10 suppresses high-frequency noise. Each component performs its function, working together to achieve high-precision constant current control.

[0039] Figure 2 A schematic diagram of the structure of the constant current source circuit 1 according to the second embodiment of this disclosure is shown. Figure 2 The illustrated embodiments and Figure 1 The embodiments shown are similar; the focus is on describing their differences. In some embodiments, such as Figure 2As shown, the isolation and sampling circuit 30 includes a third operational amplifier U3 as an isolator. The third operational amplifier U3 has an input bias current on the order of fA (e.g., 0.1fA to tens of fA, e.g., 1fA) to extract current as small as 0.01pA to 10pA (e.g., 1pA) flowing through the output of the range resistor 40. It should be understood that the above numerical ranges are merely exemplary; in some embodiments, even less than 0.1pA (e.g., 0.01pA) of current can be extracted. In other embodiments, more than 10pA of current can be extracted.

[0040] In some embodiments, the third operational amplifier U3 of the isolation and sampling circuit 30 is positioned between the inverting input of the second operational amplifier U2 and the output of the range resistor 40. The input bias current of the third operational amplifier U3 is less than the input bias current of the first operational amplifier U1 and the second operational amplifier U2, and / or the input impedance of the third operational amplifier U3 is greater than the input impedance of the first operational amplifier U1 and the second operational amplifier U2. Therefore, the third operational amplifier U3 has a higher current resolution than the first operational amplifier U1 and the second operational amplifier U2. This combination of different resolutions allows for coverage of a larger current range while simultaneously providing higher current resolution.

[0041] In some embodiments, the third operational amplifier U3 is implemented as a voltmeter operational amplifier. The voltmeter operational amplifier has an extremely high input impedance (e.g., up to 10^14 Ω or higher) and extremely low input leakage current (e.g., not greater than 20 fA). In some embodiments, the third input bias current of the third operational amplifier U3 is one of the following value ranges: not greater than 80 femtoamperes, not greater than 50 femtoamperes, not greater than 30 femtoamperes, not greater than 20 femtoamperes, and not greater than 10 femtoamperes. According to this disclosure, the third operational amplifier U3 can accurately extract weak current signals with high resolution. It should be understood that the above-described input impedance and input bias current are merely exemplary.

[0042] In some embodiments, the first input bias current of the first operational amplifier U1 and / or the second input bias current of the second operational amplifier U2 are in the pA or nA range. The third input bias current of the third operational amplifier U3 is in the fA range. In some embodiments, the third input impedance of the third operational amplifier U3 is not less than one of the following values: 1TΩ, 50TΩ, or 100TΩ. It should be understood that the above input impedance and input bias current are merely exemplary.

[0043] In some embodiments, such as Figure 2 As shown, the third operational amplifier U3 is implemented as a voltage follower. This allows the voltage at the output of the range resistor to be acquired.

[0044] In some embodiments, such as Figure 1 and Figure 2 As shown, the non-inverting input of the third operational amplifier U3 is connected to the output of the range resistor 40, and the inverting input of the third operational amplifier U3 is connected to its output; the output of the third operational amplifier U3 is connected to the inverting input of the second operational amplifier U2. Therefore, the voltage obtained from the third operational amplifier U3 can be amplified.

[0045] In some embodiments, such as Figure 1 and Figure 2 As shown, the second amplifier circuit 20 is configured to amplify the output voltage at the current output terminal I_out in phase.

[0046] In some embodiments, such as Figure 1 and Figure 2 As shown, the constant current source circuit 1 also includes an inverting voltage branch 22 located at the inverting input terminal of the second operational amplifier U2. The inverting voltage branch 22 includes a first resistor R1 and a second resistor R2 connected in series. The other end of the first resistor R1 is connected to the output terminal of the second operational amplifier U2, and the other end of the second resistor R2 is connected to the output terminal of the third operational amplifier U3. The inverting input terminal of the second operational amplifier U2 is connected to the connection node between the first resistor R1 and the second resistor R2. The amplification gain of the second amplifier circuit 20 can be easily set using the first and second resistors.

[0047] In some embodiments, such as Figure 1 and Figure 2 As shown, the constant current source circuit 1 also includes a non-inverting voltage branch 24 located at the non-inverting input terminal of the second operational amplifier U2. The non-inverting voltage branch 24 includes a third resistor R3 and a fourth resistor R4 connected in series. The other end of the third resistor R3 is connected to the output terminal of the first operational amplifier U1, and the other end of the fourth resistor R4 is grounded. The non-inverting input terminal of the second operational amplifier U2 is connected to the connection node between the third resistor R3 and the fourth resistor R4. The non-inverting gain of the second amplifier circuit 20 can be easily set through the third resistor and the fourth resistor.

[0048] According to this disclosure, weak current signals can be accurately extracted through isolation and sampling circuits (for example, the isolation and sampling circuits can be implemented as electrometer operational amplifiers, which utilize their fA-level input bias current (≤20fA) and ultra-high input impedance (≥10^14Ω) characteristics to accurately extract the weak voltage signal at the output of the feedback resistor without "stealing" the weak current signal).

[0049] According to this disclosure, the three functional levels are clearly defined: the third-level electrometer tracks and extracts weak current, the second-level proportional amplifier adjusts the gain, and the third-level integral amplifier suppresses high-frequency noise. Each level performs its own function and works together to achieve high-precision constant current control.

[0050] According to this disclosure, the second-stage output is connected back to the inverting terminal of the first stage to form a closed-loop negative feedback, thereby achieving precise constant current control, high output current stability, and stable closed-loop feedback.

[0051] According to this disclosure, full-range compatibility can be achieved. This three-level architecture can work stably across the entire range of pA / nA / μA / mA, simplifying the circuit structure.

[0052] According to this disclosure, a signal processing circuit is also provided, including the constant current source circuit described in the foregoing aspects.

[0053] Although the subject matter has been described using language specific to structural features and / or methodological logic, it should be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or actions described above. Rather, the specific features and actions described above are merely illustrative examples of implementing the claims.

[0054] The various embodiments of this disclosure have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or technical improvements to the embodiments in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A constant current source circuit (1), comprising: Voltage input terminal (V_set); Current output terminal (I_out); A range resistor (40) is connected to the current output terminal (I_out) of the constant current source circuit. A first amplifier circuit (10) is configured to provide an output current based on the input voltage at the voltage input terminal (V_set), and includes a first operational amplifier (U1), the non-inverting input terminal of which is connected to the voltage input terminal (V_set) of the constant current source circuit and configured to receive the input voltage at the voltage input terminal (V_set), wherein an integrating capacitor (C) is provided between the output terminal of the first operational amplifier (U1) and the inverting input terminal of the first operational amplifier (U1); The second amplifier circuit (20) is configured to proportionally amplify the output voltage at the current output terminal (I_out), and includes a second operational amplifier (U2), the output of which is connected to the inverting input of the first operational amplifier (U1); and An isolation and sampling circuit (30) is disposed between the inverting input of the second operational amplifier (U2) and the output of the range resistor (40) and is configured to extract the output voltage acquired at the output of the range resistor (40), wherein the second operational amplifier (U2) is configured to amplify the acquired output voltage proportionally.

2. The constant current source circuit according to claim 1, wherein the isolation and sampling circuit (30) includes a third operational amplifier (U3) disposed between the inverting input terminal of the second operational amplifier (U2) and the output terminal of the range resistor (40), wherein the input bias current of the third operational amplifier (U3) is less than the input bias current of the first operational amplifier (U1) and the second operational amplifier (U2) and / or the input impedance of the third operational amplifier (U3) is greater than the input impedance of the first operational amplifier (U1) and the second operational amplifier (U2).

3. The constant current source circuit according to claim 2, wherein the third operational amplifier (U3) is implemented as a voltage follower.

4. The constant current source circuit according to claim 1, wherein the isolation and sampling circuit (30) includes a third operational amplifier (U3) as an isolator disposed between the inverting input terminal of the second operational amplifier (U2) and the output terminal of the range resistor (40), wherein the third operational amplifier (U3) is implemented as an electrometer operational amplifier.

5. The constant current source circuit according to claim 4, wherein the third operational amplifier (U3) has an input bias current on the order of fA to be able to extract a current as small as 0.01pA to 1pA flowing through the output terminal of the range resistor (40).

6. The constant current source circuit according to any one of claims 2-5, wherein the non-inverting input terminal of the third operational amplifier (U3) is connected to the output terminal of the range resistor (40), the inverting input terminal of the third operational amplifier (U3) is connected to the output terminal of the third operational amplifier (U3), and the output terminal of the third operational amplifier (U3) is connected to the inverting input terminal of the second operational amplifier (U2).

7. The constant current source circuit according to any one of claims 1-5 further includes an inverting voltage branch (22) located at the inverting input terminal of the second operational amplifier (U2), the inverting voltage branch (22) including a first resistor (R1) and a second resistor (R2) connected in series with each other, the other end of the first resistor (R1) being connected to the output terminal of the second operational amplifier (U2), the other end of the second resistor (R2) being connected to the output terminal of the third operational amplifier (U3), and the inverting input terminal of the second operational amplifier (U2) being connected to the connection node between the first resistor (R1) and the second resistor (R2).

8. The constant current source circuit according to any one of claims 1-5 further includes a non-inverting voltage branch (24) located at the non-inverting input terminal of the second operational amplifier (U2), the non-inverting voltage branch (24) including a third resistor (R3) and a fourth resistor (R4) connected in series, the other end of the third resistor (R3) being connected to the output terminal of the first operational amplifier (U1), the other end of the fourth resistor (R4) being grounded, and the non-inverting input terminal of the second operational amplifier (U2) being connected to the connection node between the third resistor (R3) and the fourth resistor (R4).

9. The constant current source circuit according to any one of claims 2-5, wherein the first operational amplifier (U1) has a first input bias current, the second operational amplifier (U2) has a second input bias current, the third operational amplifier (U3) has a third input bias current, and the third input bias current is less than either the first input bias current or the second input bias current.

10. The constant current source circuit according to claim 9, wherein... The first input bias current and / or the second input bias current are in the pA or nA range; the third input bias current is in the fA range; and / or The third input bias current is one of the following value ranges: not greater than 80 femtamps, not greater than 50 femtamps, not greater than 30 femtamps, not greater than 20 femtamps, and not greater than 10 femtamps.

11. The constant current source circuit according to any one of claims 2-5 and 10, wherein... The first operational amplifier (U1) has a first input impedance, the second operational amplifier (U2) has a second input impedance, and the third operational amplifier (U3) has a third input impedance, wherein the third input impedance is greater than the first input impedance and the second input impedance; and / or the third input impedance is not less than one of the following values: 1TΩ, 50TΩ, 100TΩ.

12. The constant current source circuit according to any one of claims 1-5, 10, wherein the range resistor (40) comprises a plurality of resistors having different resistance values ​​from each other; The constant current source circuit also includes a switch (S) arranged between the range resistor (40) and the output of the first operational amplifier (U1), the switch being configured to selectively connect one of the plurality of resistors of the range resistor (40) to the output circuit of the constant current source circuit.

13. The constant current source circuit of claim 12, wherein the plurality of resistors includes a first range resistor with a resistance value selected from one of the following ranges: 0.1 GΩ to 800 GΩ, 1 GΩ to 500 GΩ, 10 GΩ to 100 GΩ; and / or the plurality of resistors includes a second range resistor with a resistance value selected from one of the following ranges: 1 KΩ to 10 MΩ.

14. A signal processing circuit comprising a constant current source circuit according to any one of claims 1-13.

Images