Low Power Voltage Detector for Low Voltage CMOS Processes

Abstract

The voltage detector has a diode ladder in which one or more diodes are connected in series between the battery voltage input and the upper measurement node. The measurement diode is connected between the upper measurement node and the lower measurement node. A resistor and a power-off switch are connected in series between the lower measurement node and ground. The analog input of the analog-to-digital converter (ADC) is connected to the upper measurement node through a switch to generate an upper digital value. The switch then connects the analog input to the lower measurement node to generate a lower digital value. The difference between the upper and lower digital values ​​is the diode voltage drop across the measurement diode, which is multiplied by the number of diodes in the diode ladder and added to the upper digital value to produce a battery voltage measurement value.

Description

Technical Field

[0001] The present invention relates to a voltage detector, and more particularly to a battery voltage detector for measuring a diode terminal voltage. Background Art

[0002] Battery-powered devices may include integrated circuits (ICs) containing semiconductor devices such as transistors. Transistors produced by more advanced semiconductor processes may be damaged by higher applied voltages. Smaller batteries can power advanced low-power ICs that have lower supply voltages, and transistors may also be damaged by voltages as low as the battery voltage. Therefore, the battery voltage must be reduced or applied to several transistors in series to reduce the voltage applied to any single transistor.

[0003] FIG1 shows a resistor divider. Battery 20 generates a battery voltage VBAT at its positive terminal relative to its ground terminal. Between VBAT and ground, resistors 22, 24 are connected in series with each other and with switch 26. When switch 26 is closed, current I from battery 20 passes through resistors 22, 24, causing a voltage drop V=IR across each of resistors 22, 24 that is proportional to the resistance R of each resistor. When resistors 22, 24 have the same resistance R, the node between resistors 22, 24, voltage VIN, is half the battery voltage, or VIN=VBAT / 2.

[0004] When switch 26 is closed and VBAT is divided by resistors 22, 24, VIN can be safely applied to transistors or other precision semiconductor devices. For example, VIN can be used to indirectly measure the battery voltage. However, the current I through resistors 22, 24 may cause unnecessary power consumption of battery 20. When the battery voltage is not measured, switch 26 can be opened to interrupt the current flow and reduce power.

[0005] However, when switch 26 is open, current from battery 20 will momentarily flow through resistor 22 and charge VIN until VIN reaches VBAT. When switch 26 is open, any transistors connected to VIN have a higher VBAT voltage. These transistors may be damaged when switch 26 is open because VIN may be equal to or close to VBAT.

[0006] For example, VBAT may be 1.5 to 4.2 volts, which would put VIN in the safe range of 0.75 to 2.1 volts when switch 26 is closed and equal resistors R are used, but VIN is in the range of 1.5 to 4.2 volts when switch 26 is open. Some transistors or other devices may be damaged by 3 or 4 volts. Therefore, a resistor divider with a power-off switch may damage sensitive transistors or other devices.

[0007] A diode voltage divider is shown in Figure 2. A series of diodes 30, 32, 34, 36 are connected in series with each other and with a switch 38. When the switch 38 is closed, the current flowing through each diode 30, 32, 34, 36 causes a pn junction voltage drop Vpn of about 0.5-0.7 volts for each diode.

[0008] In FIG. 2 , VIN is located between diodes 32 , 34 , and there is a diode drop between VBAT and VIN caused by diodes 30 , 32 , so VIN = VBAT −2*Vpn, or about 1 to 1.4 volts below VBAT.

[0009] When switch 38 is turned off to save power, any instantaneous current flowing through diode 30 stops when the lower end of diode 30 rises above VBAT – Vpn. Similarly, when the voltage across diode 32 is less than Vpn, current through diode 32 stops. Therefore, when switch 38 is turned off, VIN cannot rise above VBAT – 2*Vpn.

[0010] VIN can be connected to sensitive devices, such as transistors, because diodes 30, 32 prevent VIN from rising above VBAT-2*Vpn. When VIN is not being sampled or otherwise used, switch 38 can be opened to reduce current and power.

[0011] However, unlike resistors 22, 24 having a precise resistance value R, the voltage drop Vpn across diodes 30, 32 may vary with process, supply voltage and temperature (PVT) conditions. In addition, significant current may flow through diodes 30, 32, 34, 36, increasing power dissipation.

[0012] Low-power devices running on battery power can be in standby mode for long periods of time to extend battery life. It is important to be able to shut down the voltage detector to prevent battery drain. Advanced semiconductor processes can be so sensitive to damage that the battery voltage may cause damage. For example, a 4.2 volt battery exceeds the 3.3 volt limit of some advanced CMOS processes.

[0013] There is a need for a voltage detector that can be powered off. There is a need for a voltage detector that protects sensitive devices (such as low power transistors) from damage in power-off mode. There is a need for a voltage detector that can read the battery voltage while protecting semiconductor process transistors with low supply voltages. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 shows a resistor divider.

[0015] Figure 2 shows a diode voltage divider.

[0016] Figure 3is a block diagram of a simplified battery voltage detector that measures the voltage at two points in a diode ladder.

[0017] Figure 4 is used Figure 3 A flow chart of the battery voltage detection routine of the battery voltage detector.

[0018] Figure 5 is a block diagram of a buffered battery voltage detector that measures the voltage at two points in a diode ladder.

[0019] Figure 6 is a block diagram of a transistor-switched battery voltage detector that measures the voltage at two points in a diode ladder. DETAILED DESCRIPTION

[0020] The present invention relates to an improvement in a battery voltage detector. The following description is intended to enable one of ordinary skill in the art to make and use the invention in the context of a particular application and its requirements. Various modifications to the preferred embodiments will be apparent to those skilled in the art, and the general principles defined herein may also be applied to other embodiments. Therefore, the present invention is not intended to be limited to the specific embodiments shown and described, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

[0021] The inventors have realised that a diode ladder can provide protection for sensitive circuits by ensuring that the higher battery voltage is stepped down before being applied to the sensitive circuits. Protection can be provided even when the diode ladder is de-energised.

[0022] The inventors also realized that the diode voltage drop is sensitive to process and other PVT variations, so a single voltage measurement may not accurately measure the battery voltage. However, all diodes in a diode ladder should track each other and have the same diode voltage drop. The inventors measured the voltage across one diode in the diode ladder and then multiplied this measured diode voltage drop by the number of diodes in the diode ladder to estimate the battery voltage at the top of the diode ladder.

[0023] Figure 3 is a block diagram of a simplified battery voltage detector that measures the voltage at two points in a diode ladder. The battery voltage VBAT generated by the battery 20 is applied to the top of a diode tree consisting of diodes 30, 32, 34, 36, which are connected in series with a resistor 40 and a switch 42. The resistor 40 limits the conduction current, and the switch 42 is opened, de-energizing the battery detector.

[0024] Diode 36 has an upper terminal VD1 and a lower terminal VD0. Instead of measuring a single voltage, two voltages are measured, namely VD1 and VD0. Analog-to-digital converter (ADC) 60 measures voltage VD1 when switch 54 is closed, but measures voltage VD0 when switch 56 is closed. ADC 60 may contain delicate semiconductor components, such as precision capacitors or resistors and transistor switches, which may be damaged if the battery voltage VBAT is applied directly to the input VIN of ADC 60. The three diode voltage drops of diodes 30, 32, 34 reduce VBAT to provide a lower, safer VIN for ADC 60.

[0025] ADC 60 converts the analog input voltage VIN to digital values ​​D[7:0]. At any particular time, only one of switches 52, 54, 56 is closed. Switch 54 is closed to connect VIN to VD1, and switch 56 is closed to connect VIN to VD0. Switch 52 is closed to drive VDD to VIN to idle, reset, auto-zero, or offset calibrate ADC 60 between measurements.

[0026] By measuring the VD1 and VD0 voltages, the voltage across diode 36 can be determined by the difference of VD1-VD0. This voltage difference VD1-VD0 is the diode voltage drop. This diode voltage drop varies with process, temperature, and voltage, but should be the same for all diodes on a particular device that are manufactured together and have similar sizes, geometries, and orientations. Therefore, when diodes 30, 32, 34, 36 are designed to have the same size, geometry, and orientation, they should have the same voltage drop when current flows through them in series.

[0027] The total voltage drop across the upper diodes 30, 32, 34 is three times the measured voltage drop across diode 36, i.e., 3x(VD1-VD0). Since VD1 is the voltage between diodes 34 and 36, and the battery voltage VBAT is the voltage at the top of diode 30, VBAT can be calculated as:

[0028] VBAT=3x(VD1-VD0)+VD1

[0029] Generally speaking, when there are N diodes between VBAT and VD1,

[0030] VBAT=N x(VD1-VD0)+VD1

[0031] Thus, the battery voltage can be determined from the two voltage measurements VD1 and VD0. The battery voltage is obtained by multiplying the voltage drop across the diode 36 by the number of diodes between the diode 36 and the battery 20.

[0032] Figure 4 is used Figure 3 FIG. 2 is a flow chart of a battery voltage detection routine of a battery voltage detector of FIG. 201 . In step 202 , when the voltage detector is instructed to power on and take a measurement, switch 42 is closed by the power-on signal PUP going high. When the signal MXH is high, switch 52 is closed to drive VDD to ADC 60 to preset it, while signals MH1 and MX0 are low to keep switches 54 and 56 open.

[0033] After the power-up delay PUP ends (step 204), the switches 52, 54, 56 controlled by the signals MXH, MX1, MX0 respectively change to switches 52, 56 open and switch 54 closed to connect VD1 to VIN and ADC 60 (step 212). In step 212, MX1 is high, and MXH and MX0 are low.

[0034] ADC 60 measures VIN=VD1 and converts the analog voltage into a digital value D[7:0] (step 216), which is stored as digital voltage DV1.

[0035] MX0 is then driven high and MX0 and MXH are driven low (step 222) to close switch 56 and open switches 52, 54. ADC 60 measures VIN=VD0 and converts the analog voltage to a digital value D[7:0] (step 226), which is stored as digital voltage DV0.

[0036] The arithmetic logic unit (ALU) or other microprocessor or programmed logic then calculates the measured battery voltage VBAT as a digital value DVBAT (step 230). The difference DV1-DV0 is multiplied by the number of diodes N between VD1 and VBAT and then added to DV1, or

[0037] DVBAT=N×(DV1-DV0)+DV1

[0038] for Figure 3 The logic circuit, where N is 3:

[0039] DVBAT=3x(DV1-DV0)+DV1

[0040] The digital value DVBAT of the battery voltage calculated from the two voltage measurements on the diode 36 can then be stored (step 232). This value DVBAT can be compared with a target, such as a voltage limit, for issuing a power-off signal when DVBAT is below a lower limit, or for issuing an overvoltage alarm when DVBAT is above an upper limit.

[0041] The battery voltage detector may then be powered down by driving PUP low (not shown) to open switch 42, or left open to repeat the measurement.

[0042] Figure 5 is a block diagram of a buffered battery voltage detector that measures the voltage at two points of a diode ladder. In this alternative, the analog input of ADC 60 is buffered by an operational amplifier 70. Operational amplifier 70 receives VIN at its non-inverting (+) input and feeds its output VINA back to its inverting (-) input. Operational amplifier 70 may be a unity gain buffer.

[0043] The operational amplifier 70 buffers VIN, which may experience voltage changes due to charge sharing when switches 52, 54, 56 are switched. The input of ADC 60 may have small sensitive capacitance. The input VINA of ADC 60 is protected by the operational amplifier 70 from charge sharing and switching noise on VIN.

[0044] Figure 6 is a block diagram of a transistor switch battery voltage detector that measures the voltage at two points in a diode ladder. In this alternative, switch 42 is implemented by transistor 78. Transistor 78 may be an n-channel MOS transistor whose gate is driven by a power-up signal PUP and has a channel between resistor 40 and ground. N-channel MOS transistors have very low leakage current and provide good isolation when the switch is closed.

[0045] Switches 52, 54, 56 are also implemented by n-channel transistors. The gate of transistor 72 is driven by control signal MXH, and the channel of transistor 72 connects VDD to VIN. The gate of transistor 74 is driven by control signal MX1, and its channel connects VD1 to VIN. The gate of transistor 76 is driven by control signal MX0, and its channel connects VD0 to VIN. The control signals MXH, MX1, MX2 can be generated by controller 90, or can even be software controlled, such as by software writing a 1 or 0 to an I / O register that drives MXH, MX1, MX0.

[0046] Also in this alternative, diode 32 has been deleted so that there are only two diodes 30, 34 located between the measuring diode 36 and VBAT. In this variant, N=2, VBAT can be calculated by the controller 90 or by a processor or other component as:

[0047] VBAT=2x(VD1-VD0)+VD1

[0048] Alternative Embodiments

[0049] The inventors have added several other embodiments. For example, an additional diode or a transistor connected to a diode can be inserted, such as in series between the diode 36 and the resistor 40. The order of the series elements can be adjusted, such as having the resistor 40 connected to the ground and the switch 42 connected between the upper end of the resistor and the diode 36.

[0050] Although unipolar transistor switches have been shown, multipolar switches, multiplexers, or switch networks can be used to connect VD0, VD1, or VDD to the ADC. These more complex switches may themselves be composed of multiple simple switches and various logic or controls. Therefore, a single multipolar switch may actually be equivalent to multiple unipolar switches. Each switch can be implemented as an n-channel transistor, or it can be a transmission gate, each with an n-channel and a p-channel transistor in parallel. Although n-channel transistors have been described, p-channel transistors can also be used, used with inverted gate signals.

[0051] There may be different numbers of diodes between the measuring diode 36 and VBAT, for example Figure 3 and Figure 5 The three diodes 30, 32, 34 shown in FIG. Figure 6 Only two diodes 30, 34 are shown. The upper diode ladder may have N diodes in series between VBAT and VD1, where N is an integer of at least one.

[0052] Although a battery voltage detector has been described, the voltage detector may measure other voltages not generated by a battery. The battery voltage may be generated by multiple battery cells in a battery or by multiple batteries. The voltage detector may measure a power supply voltage generated by a power supply that may be switched from a battery to a transformer or other power converter.

[0053] Diodes 30, 32, 34, 36 may be pn semiconductor diodes in which the upper end is connected to a p-type doped region adjacent to an n-type doped region in the same semiconductor substrate, such as in an integrated circuit (IC) chip. Diodes formed in the same semiconductor substrate, when their design and geometry are matched, should have the same pn junction doping profile and thus closely matched device characteristics, such as forward biased diode voltage drop. Diodes under forward bias generally follow the ideal diode equation, where the forward current increases exponentially with increasing voltage near the diode conduction voltage. This exponential current can be approximated as an on-off switch with a pn junction turn-on voltage drop. This pn junction voltage when turned on can be seen in a graph as the knee voltage where the current-voltage curve bends sharply upward.

[0054] Although a complementary metal oxide semiconductor (CMOS) process has been described, other processes and materials may be used. Different semiconductor materials will have different pn junction turn-on voltages. For example, the pn junction voltage of silicon is 0.5-0.7 volts, but the pn junction voltage of gallium arsenide is lower, about 0.3 volts. Since all diodes 30, 32, 34, 36 are made of the same semiconductor substrate, they will all have the same pn junction turn-on voltage and have the same diode voltage drop.

[0055] Although a pn junction diode has been described, other diode technologies may be used such as Schottky diodes, metal ohmic contacts, etc. Many variations of IC semiconductor manufacturing processes are possible.

[0056] Terms such as above, below, on, below, horizontal, vertical, inside, outside, etc. are relative and dependent on the viewing angle and are not meant to limit the invention to a particular viewing angle. The device can be rotated so that vertical is horizontal and horizontal is vertical, so these terms are observer dependent. A line or direction is considered vertical and the other line or direction is considered horizontal as long as the two directions are substantially perpendicular to each other.

[0057] Although an ADC has been described that converts an analog voltage to a digital value, an analog current can also be converted to a digital value. The analog current can be passed through a resistor to produce an analog voltage input to the ADC. Alternatively, the ADC can convert an analog current as its input. A voltage-controlled current source can convert an analog voltage input to an analog current input for such an ADC. Many kinds and variations of ADCs are possible.

[0058] Figure 4 The process may be performed by a program (such as a software program) executing on a computer or processor, or by a firmware routine, or by hardwired logic (such as a sequencer or state machine), or by various combinations. For example, the calculations in step 230 may be performed by a higher level microprocessor, while the hardware control of the switches 52, 54, 56 and the activation of the ADC 60 may be handled by hardware.

[0059] Figure 4 Some steps in the process can be performed in parallel with other steps, or in a different order or sequence. For example, VD0 can be measured before VD1. A second ADC with independent switches can measure VD0 and VD1 simultaneously.

[0060] Although the use of an ADC to measure the voltage drop across one diode is shown, more measurements may be taken and the results averaged, or outliers removed. For example, if another switch connects VIN to this node, ADC 60 may also measure the voltage across diodes 32, 34. The measured voltage drops of diodes 34 and 36 may be averaged together and then used as the diode voltage drop in further calculations.

[0061] Although switch 52 has been shown as connecting VIN to a supply voltage VDD, switch 52 may alternatively connect VIN to ground, or to a common mode voltage VCM of VDD / 2, or to some other voltage. Some embodiments may eliminate switch 52, or ADC 60 may include an internal switch to precharge or initialize its internal analog input. ADC 60 may be a switched capacitor ADC, a flash ADC, or another type of ADC. The number of digital bits in the output of ADC 60 has been described as 8, but other bit widths or resolutions, such as 6 bits, 10 bits, etc., may be used instead. The battery voltage may have a variety of different values, and additional components such as power supply capacitors, diodes, power switches, displays, or power supply filters may be connected to battery 20.

[0062] The background section of the present invention may contain background information about the problem or environment of the present invention, rather than describing the prior art of others. Therefore, the materials contained in the background section are not an admission by the applicant that the prior art is available.

[0063] Any method or process described herein is machine-implemented or computer-implemented, intended to be performed by a machine, computer or other device, and is not intended to be performed solely by humans without machine assistance. Tangible results produced may include reports or other machine-generated displays on display devices such as computer monitors, projection devices, audio generating devices, and related media devices, and may include hard copy printouts that are also machine-generated. Computer control of other machines is another tangible result.

[0064] Any advantages and benefits described do not necessarily apply to all embodiments of the invention. When the word "means" is used, it is intended that the claim element falls within the provisions of 35 USC § 112(6). Typically, the word "means" is preceded by a label of one or more words. The one or more words preceding the word "means" are a label for the purpose of facilitating reference to the claim element and are not intended to express a structural limitation. Such a means-plus-function claim is intended to cover not only the structures described herein for performing the function and their structural equivalents, but also equivalent structures. For example, although a nail and a screw have different constructions, they are equivalent structures because they both perform the function of fastening. Claims that do not use the word "means" do not fall within the provisions of 35 USC § 112(6). Signals are typically electronic signals, but can also be optical signals, such as those that can be transmitted over fiber optic lines.

[0065] The above description of the embodiments of the present invention is presented for the purpose of illustration and description. It is not intended to be exhaustive, nor is it intended to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teachings. It is intended that the scope of the present invention is not limited by this detailed description, but rather by the limitations of the claims appended hereto.

Claims

1. A voltage detector, comprising: a diode ladder having one or more diodes connected in series between a voltage input and an upper measurement node; a measuring diode connected between the upper measuring node and the lower measuring node; a resistor and a disconnect switch connected in series between the lower measurement node and ground; an analog-to-digital converter having an analog input and a digital output, the digital value of the digital output representing the voltage of the analog input; a first switch connecting the upper measurement node to the analog-to-digital converter, the upper digital value generated by the analog-to-digital converter representing the voltage of the upper measurement node; a second switch connecting the lower measurement node to the analog-to-digital converter, the analog-to-digital converter generating a lower digital value representing the voltage of the lower measurement node; as well as a controller that generates a calculated voltage measurement by calculating a difference between the upper digital value and the lower digital value, multiplying the difference by the number of diodes in the diode ladder to generate a product, and adding the product to the upper digital value, The voltage across the measuring diode is thereby measured and used to calculate the calculated voltage measurement value.

2. The voltage detector of claim 1, wherein the diode ladder comprises an upper diode and a lower diode connected in series; The controller multiplies the difference by 2 to generate the product.

3. The voltage detector of claim 1 , wherein the diode ladder comprises an upper diode and a middle diode and a lower diode connected in series; The controller multiplies the difference by 3 to generate the product.

4. The voltage detector according to claim 3, wherein: The one or more diodes in the diode ladder and the measuring diode each include a pn junction diode, wherein the one or more diodes in the diode ladder and the measuring diode are formed together in a semiconductor substrate.

5. The voltage detector according to claim 4, wherein: The diode or diodes in the diode ladder and the measuring diode are matched during design to have the same size and geometry.

6. The voltage detector according to claim 5, wherein: One or more diodes in the diode ladder and the measurement diode are forward biased by the voltage input and ground.

7. A voltage measurement circuit, comprising: Voltage input; a plurality of N diodes, wherein N is the number of diodes connected in series between the voltage input and the first node; a measuring diode connected between the first node and the second node; a resistor connected between the second node and a third node; a power-off switch connected between the third node and ground, wherein when no voltage measurement is performed, the power-off switch is disconnected to power off the voltage measurement circuit; an analog-to-digital converter that converts a voltage at an analog input node into a digital value representing the voltage at the analog input node; a first switch that connects the first node to a merging node when the digital value generated by the analog-to-digital converter is a first digital value representing a voltage of the first node; a second switch that connects the second node to the merging node when the digital value generated by the analog-to-digital converter is a second digital value representing the voltage of the second node; as well as a processor that receives the first digital value and the second digital value from the analog-to-digital converter and generates a digital estimate of the voltage of the voltage input, the processor subtracting the second digital value from the first digital value to generate a digital difference value, multiplying the digital difference value by N to generate a product, and adding the first digital value to the product to generate a digital estimate of the voltage of the voltage input, Thus, the voltage difference across the measurement diode is multiplied by the number of diodes in the plurality of N diodes and added to the first digital value to generate a digital estimate of the voltage of the voltage input.

8. The voltage measurement circuit of claim 7, wherein the merging node is directly connected to the analog input node of the analog-to-digital converter.

9. The voltage measurement circuit according to claim 7, further comprising: An analog buffer has the merging node as an input, and the analog input node of the analog-to-digital converter serves as an output of the analog buffer. 10 . The voltage measurement circuit of claim 7 , wherein each diode of the plurality of N diodes and the measurement diode each comprises a pn junction diode formed in a semiconductor substrate together with other diodes of the plurality of N diodes.

11. The voltage measurement circuit of claim 10, wherein each diode of the plurality of N diodes and the measurement diode is forward biased by the voltage input, and a voltage drop of each diode has a same value within 5%.

12. The voltage measurement circuit of claim 7, wherein the analog-to-digital converter has a precision of at least 5 bits, wherein a digital value generated by the analog-to-digital converter comprises at least 5 binary bits.

13. The voltage measurement circuit according to claim 7, further comprising: A third switch connects a fixed voltage node to the analog-to-digital converter, wherein the fixed voltage node has a fixed voltage.

14. The voltage measurement circuit according to claim 7, further comprising: An amplifier has its input connected to the merging node, and an output of the amplifier is connected to the analog input node of the analog-to-digital converter.

15. The voltage measurement circuit of claim 14, wherein the amplifier is an operational amplifier, an inverting input of the operational amplifier being connected to an output of the amplifier.

16. The voltage measurement circuit according to claim 7, wherein the first switch comprises a first n-channel transistor, a channel of which is connected between the first node and the merging node, and a control gate of which is used to isolate the first node from the merging node; The second switch includes a second n-channel transistor, a channel of which is connected between the second node and the merging node, and a control gate of which is used to isolate the second node from the merging node.

17. The voltage measurement circuit according to claim 7, further comprising: a battery connected to said voltage input; The power-off switch is disconnected to power off the voltage measurement circuit, thereby saving battery power.

18. The voltage measurement circuit of claim 17, wherein the power-off switch comprises an n-channel transistor having a channel connected in series with the resistor and a control gate thereof receiving a power mode signal, wherein when the power mode signal indicates a power-off mode, the n-channel transistor does not conduct current from the resistor.

19. A voltage measurement and protection device, comprising: an analog-to-digital converter that converts an analog input node into a digital value representing a voltage at the analog input node; wherein the digital value has at least 5 binary bits; a voltage input for receiving an input voltage that would damage the analog-to-digital converter if applied directly to an analog input node; a series of N diodes connected in series between the input voltage and a first node, each diode in the series of N diodes being forward biased by a current from the voltage input, wherein N is an integer; a measuring diode connected between the first node and the second node; a resistor and a disconnect switch connected in series between the second node and ground; a first switch that connects the first node to a merging node when the digital value generated by the analog-to-digital converter represents a first digital value of the voltage of the first node; a second switch that connects the second node to the merging node when the digital value generated by the analog-to-digital converter represents a second digital value of the voltage of the second node; a buffer connected between the merging node and the analog input node of the analog-to-digital converter; as well as an estimator that receives the first digital value and the second digital value from the analog-to-digital converter and generates a digital estimate of the voltage of the voltage input, the estimator subtracting the second digital value from the first digital value to generate a digital difference value, multiplying the digital difference value by N to generate a product, and adding the first digital value to the product to generate the digital estimate of the voltage of the voltage input.

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