A fully differential ADC driver circuit

Abstract

The present invention discloses a fully differential ADC driving circuit, which supports a single-ended input differential output mode and a dual-ended input differential output mode. The circuit comprises a forward input amplification module, a reverse input amplification module, an intermediate mode conversion module, and a signal extraction module. The forward input amplification module is used to configure a differential signal forward input amplification factor; the reverse input amplification module is used to configure a differential signal reverse input amplification factor; the intermediate mode conversion module is used to implement a single-ended input working mode through signal reverse processing and bias voltage input; and the signal extraction module is used to extract a reverse input signal in the single-ended mode.

Description

Technical Field

[0001] The present invention belongs to the technical field of drive circuits, and in particular relates to a fully differential ADC drive circuit. Background Art

[0002] In the field of data acquisition, the demand for high-gain bandwidth, low-noise differential operational amplifiers is growing. However, differential amplifiers are mainly imported. Domestic chips have insufficient noise and bandwidth, and it is difficult to find corresponding domestic material substitutes during product development. As a result, product development and supply delivery of core components rely on imports, facing procurement difficulties and rising procurement costs.

[0003] The purpose of this application is to provide a fully differential drive circuit that supports both single-ended input and differential output modes and dual-ended input and differential output modes, with simple peripheral parameter settings, to implement a high-gain-bandwidth, low-noise differential operational amplifier. By using multiple single-ended op amps to implement a circuit that can replace imported differential op amps or partially replace differential op amps, this circuit indirectly achieves domestic substitution of high-gain-bandwidth, low-noise ADC fully differential op amps. Summary of the Invention

[0004] The purpose of the present invention is to design and provide a fully differential ADC driver circuit to achieve the localization and universalization of differential drive. To achieve this purpose, the specific technical solutions of this application are as follows:

[0005] A fully differential ADC driver circuit supports single-ended input differential output mode and double-ended input differential output mode, including a forward input amplification module, a reverse input amplification module, an intermediate mode conversion module and a signal extraction module.

[0006] The forward input amplification module is used to configure the differential signal forward input amplification factor, and its two ends are respectively connected to the forward input terminal of the driving circuit and the intermediate mode conversion module, and the forward input terminal of the driving circuit inputs the forward input signal VIp;

[0007] The reverse input amplification module is used to configure the reverse input amplification factor of the differential signal, and its two ends are respectively connected to the reverse input terminal of the driving circuit and the intermediate mode conversion module, and the reverse input terminal of the driving circuit inputs the reverse input signal VIn;

[0008] The intermediate mode conversion module is used to realize a single-ended input working mode by performing signal reverse processing and bias voltage input. The module is respectively connected to the forward input amplification module, the reverse input amplification module, the bias voltage input terminal, the reverse output terminal of the driving circuit, and the forward output terminal of the driving circuit. The signal extraction module is connected between the intermediate mode conversion module and the reverse output terminal and the forward output terminal of the driving circuit. The bias voltage input terminal is connected to a reference source Vcom.

[0009] The signal extraction module is used to extract the reverse input signal in the single-ended mode, and is respectively connected to the intermediate mode conversion module, the reverse output end of the driving circuit and the forward output end of the driving circuit. The reverse output end of the driving circuit outputs the reverse output signal Von, and the forward output end of the driving circuit outputs the forward output signal Vop.

[0010] Furthermore, the forward input amplification module includes an operational amplifier U1, the forward input terminal of the operational amplifier U1 is connected to the forward input terminal of the driving circuit, the reverse input terminal of the operational amplifier U1 is connected to the resistor R1 and then grounded, the output terminal of the operational amplifier U1 is connected to the resistor R5, and the resistor R2 is connected in series between the output terminal and the reverse input terminal of the operational amplifier U1.

[0011] Furthermore, the reverse input amplification module includes an operational amplifier U2, the positive input terminal of the operational amplifier U2 is connected to the reverse input terminal of the driving circuit, the reverse input terminal of the operational amplifier U2 is connected to the resistor R3 and then grounded, the output terminal of the operational amplifier U2 is connected to the resistor R8, and the resistor R4 is connected in series between the output terminal and the reverse input terminal of the operational amplifier U2.

[0012] Furthermore, the intermediate mode conversion module includes an operational amplifier U3 and an operational amplifier U4, wherein the positive input terminal of the operational amplifier U3 is connected in parallel with a resistor R9 and a resistor R11, the other end of the resistor R9 is connected to the bias voltage input terminal, and the resistor R11 is grounded; the negative input terminal of the operational amplifier U3 is connected to a resistor R7, and the other end of the resistor R7 is connected to a resistor R5; a resistor R13 is connected in series between the output terminal and the negative input terminal of the operational amplifier U3;

[0013] The positive input terminal of the operational amplifier U4 is connected in parallel with resistors R10 and R12, the other end of resistor R10 is connected to the bias voltage input terminal, and the other end of resistor R12 is grounded; the negative input terminal of the operational amplifier U4 is connected to resistor R8, and the other end of resistor R8 is connected to resistor R6; a resistor R14 is connected in series between the output terminal and the negative input terminal of the operational amplifier U4.

[0014] Furthermore, the signal extraction module includes an operational amplifier U5 and an operational amplifier U6, wherein the positive input terminal of the operational amplifier U5 is connected in parallel with resistors R17 and R18, the other end of the resistor R17 is grounded, and the other end of the resistor R18 is connected to the output terminal of the operational amplifier U3; the negative input terminal of the operational amplifier U5 is connected in parallel with resistors R20 and R21, the other end of the resistor R20 is connected to the bias voltage input terminal, and the other end of the resistor R21 is grounded; the output terminal of the operational amplifier U5 is connected in series with resistor R16 and then connected to resistor R6; and a resistor R19 is connected in series between the negative input terminal and the output terminal of the operational amplifier U5;

[0015] The positive input terminal of the operational amplifier U6 is connected in parallel with resistors R25 and R26, the other end of resistor R25 is connected to the positive output terminal of the drive circuit, and the other end of resistor R26 is grounded; the negative input terminal of the operational amplifier U6 is connected in parallel with resistors R23 and R24, the other end of resistor R23 is connected to the bias voltage input terminal of the drive circuit, and the other end of resistor R24 is grounded; the output terminal of the operational amplifier U6 is connected in series with resistor R15 and then connected to resistor R5; and resistor R22 is connected in series between the negative input terminal and the output terminal of the operational amplifier U6.

[0016] Furthermore, when the driving circuit is in a dual-end input differential output mode, the resistors R15 and R16 are in an open circuit state, disconnecting the signals output from the operational amplifiers U5 and U6 to the operational amplifiers U4 and U3.

[0017] Furthermore, when the driving circuit is in a single-ended input and differential output mode, an analog signal is input to the positive input terminal of the driving circuit, resistor R15 is disconnected, resistor R16 is connected, resistor R15 connects the signal output from operational amplifier U5 to operational amplifier U4, resistor R16 = 0Ω, and VIp is grounded.

[0018] Furthermore, when the driving circuit is in a single-ended input and differential output mode, an analog signal is input to the reverse input terminal of the driving circuit, resistor R16 is disconnected, resistor R15 is connected, resistor R16 connects the signal output from operational amplifier U6 to operational amplifier U3, resistor R16 = 0Ω, and VIn is grounded.

[0019] Furthermore, the feedback networks of the forward input amplification module and the reverse input amplification module are used to adjust the gain parameters; while the intermediate mode conversion module does not need to adjust the gain, and the feedback network and other parameters involved are fixed.

[0020] Furthermore, the driving circuit can be integrated into a gain-adjustable driving module, which includes a positive input interface VIp, a reverse input interface VIn, a bias voltage input interface Vcom, a reverse output interface Von, a positive output interface Vop, a power supply interface VCC, a negative power supply interface VEE, a feedback network interface FG, a feedback network interface FB and an MS interface, and the MS interface is used to set the resistance when single-ended input is used.

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] (1) The structure of this application can be fully domestically produced, and a high-gain, broadband, low-noise differential operational amplifier can be realized with a low-cost circuit structure;

[0023] (2) The driving circuit of this application supports single-ended input differential output mode and double-ended input differential output mode;

[0024] (3) Setting up a signal extraction module can remove the DC bias from the output signal of the forward input when there is a DC bias;

[0025] (4) The gain parameter of the positive input signal of the driving circuit is adjusted directly by adjusting the resistors R1 and R2, and the gain parameter of the reverse input signal of the driving circuit is adjusted by adjusting the resistors R3 and R4;

[0026] (5) The drive circuit can be integrated into a gain-adjustable drive module, with an external interface reserved for users. The peripheral parameter settings are simple. For basic applications, only the resistance values of the peripheral resistors R1, R2, R3, R4, R15 and R16 involved in the amplification factor need to be adjusted, and the remaining resistors can be fixed. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a schematic diagram of the single-ended input and differential output mode of the present invention;

[0028] Figure 2 This is a schematic diagram of the dual-end input and differential output mode of the present invention;

[0029] Figure 3 A module diagram of a driving circuit of the present invention;

[0030] Figure 4 A circuit diagram of a driving circuit of the present invention;

[0031] Figure 5 This is a diagram of the driving module of the present invention. DETAILED DESCRIPTION

[0032] In order to enable those skilled in the art to better understand the technical solution of the present invention, a fully differential ADC driving circuit of the present invention is further described below with reference to the accompanying drawings.

[0033] like Figure 1-2 The figure shows the use of the present invention, which supports single-ended input differential output mode and double-ended input differential output mode. The input end selects analog differential signal or analog single-ended signal to achieve differential amplification output.

[0034] like Figure 3-4As shown, the fully differential ADC driving circuit includes a forward input amplification module, a reverse input amplification module, an intermediate mode conversion module and a signal extraction module. The forward input amplification module is used to configure the differential signal forward input amplification factor, and its two ends are respectively connected to the forward input end of the driving circuit and the intermediate mode conversion module. The forward input end of the driving circuit inputs the forward input signal VIp; the reverse input amplification module is used to configure the differential signal reverse input amplification factor, and its two ends are respectively connected to the reverse input end of the driving circuit and the intermediate mode conversion module. The reverse input end of the driving circuit inputs the reverse input signal VIn; the intermediate mode conversion module is used to realize the single-ended input working mode when the signal is reversely processed and the bias voltage is input, and is respectively connected to the forward input amplification module, the reverse input amplification module and the intermediate mode conversion module. The intermediate mode conversion module is connected to the reverse output terminal and the forward output terminal of the driving circuit, and the signal extraction module is connected between the intermediate mode conversion module and the reverse output terminal and the forward output terminal of the driving circuit. The bias voltage input terminal is connected to the reference source Vcom. The signal extraction module is used to extract the reverse input signal in the single-ended mode and is connected to the intermediate mode conversion module, the reverse output terminal of the driving circuit and the forward output terminal of the driving circuit respectively. The reverse output terminal of the driving circuit outputs a reverse output signal Von, and the forward output terminal of the driving circuit outputs a forward output signal Vop. The feedback networks of the forward input amplification module and the reverse input amplification module are used to adjust the gain parameters. However, the intermediate mode conversion module does not need to adjust the gain, and the feedback network and other parameters involved are fixed.

[0035] Specifically, the forward input amplification module includes an operational amplifier U1, wherein the forward input of operational amplifier U1 is connected to the forward input of the drive circuit, the reverse input of operational amplifier U1 is connected to resistor R1 and then to ground, the output of operational amplifier U1 is connected to resistor R5, and a resistor R2 is connected in series between the output and reverse input of operational amplifier U1. The reverse input amplification module includes an operational amplifier U2, wherein the forward input of operational amplifier U2 is connected to the reverse input of the drive circuit, the reverse input of operational amplifier U2 is connected to resistor R3 and then to ground, the output of operational amplifier U2 is connected to resistor R8, and a resistor R4 is connected in series between the output and reverse input of operational amplifier U2. The intermediate mode conversion module includes operational amplifiers U3 and U4, wherein the forward input of operational amplifier U3 is connected in parallel with resistors R9 and R11, the other end of resistor R9 is connected to the bias voltage input, and resistor R11 is grounded; the reverse input of operational amplifier U3 is connected to resistor R7, the other end of resistor R7 is connected to resistor R5; and a resistor R13 is connected in series between the output and reverse input of operational amplifier U3. The positive input of operational amplifier U4 is connected in parallel to resistors R10 and R12, with the other end of resistor R10 connected to the bias voltage input, and the other end of resistor R12 is grounded. The negative input of operational amplifier U4 is connected to resistor R8, with the other end of resistor R8 connected to resistor R6. Resistor R14 is connected in series between the output and negative input of operational amplifier U4. The signal extraction module includes operational amplifiers U5 and U6. The positive input of operational amplifier U5 is connected in parallel to resistors R17 and R18, with the other end of resistor R17 grounded, and the other end of resistor R18 connected to the output of operational amplifier U3. The negative input of operational amplifier U5 is connected in parallel to resistors R20 and R21, with the other end of resistor R20 connected to the bias voltage input, and the other end of resistor R21 grounded. The output of operational amplifier U5 is connected in series to resistor R16 and then to resistor R6. Resistor R19 is connected in series between the negative input and output of operational amplifier U5. The positive input terminal of the operational amplifier U6 is connected in parallel with resistors R25 and R26, the other end of resistor R25 is connected to the positive output terminal of the drive circuit, and the other end of resistor R26 is grounded; the negative input terminal of the operational amplifier U6 is connected in parallel with resistors R23 and R24, the other end of resistor R23 is connected to the bias voltage input terminal of the drive circuit, and the other end of resistor R24 is grounded; the output terminal of the operational amplifier U6 is connected in series with resistor R15 and then connected to resistor R5; and resistor R22 is connected in series between the negative input terminal and the output terminal of the operational amplifier U6.

[0036] Among them, the series resistor R2 is the feedback network of the forward input amplifier module for adjusting the gain parameter of the operational amplifier U1; the series resistor R4 is the feedback network of the reverse input amplifier module for adjusting the gain parameter of the operational amplifier U2; the series resistor R13 is the feedback network of the intermediate mode conversion module for adjusting the gain parameter of the operational amplifier U3; the series resistor R14 is the feedback network of the intermediate mode conversion module for adjusting the gain parameter of the operational amplifier U4.

[0037] When the driving circuit is in dual-ended input differential output mode, resistors R15 and R16 need to be configured to the disconnected state; when the driving circuit is in single-ended input differential output mode, R15 or R16 can be configured to the connected state as needed.

[0038] Operational amplifier U1 and operational amplifier U2 form an amplification circuit for the positive input signal and the reverse input signal of the driving circuit. The amplification factor of operational amplifier U1 is determined by resistors R1 and R2, and the amplification factor of operational amplifier U2 is determined by resistors R3 and R4. In the single-ended input differential output mode, the driving circuit formed by operational amplifier U5 and operational amplifier U6 extracts the reverse amplified signal of the positive input VIp or the reverse amplified signal of the reverse input VIn from the output signal. Operational amplifier U3 and operational amplifier U4 realize the reverse amplification of the positive and negative input signals. The bias voltage input, operational amplifier U5 and operational amplifier U6 are combined to realize the single-ended input working mode. Among them, the performance of the amplifier circuit such as unit gain bandwidth and noise mainly depends on the parameters of the selected operational amplifier. The details are as follows:

[0039] (1) Dual-ended input differential output mode:

[0040] Resistors R15 and R16 are configured to be in an off state, disconnecting the signals output by operational amplifiers U5 and U6 to operational amplifiers U4 and U3;

[0041] Set R1=R3, R2=R4, the forward input amplification factor and the reverse input coefficient A are:

[0042]

[0043] or

[0044]

[0045] The output of VIp through U1 is:

[0046]

[0047] The output of VIn through U2 is:

[0048]

[0049] Set R5=R6=R7=R8, R13=R5+R7, R14=R6+R8, R9=R10=R11=R12=R13=R14, and we get

[0050]

[0051]

[0052] The differential output is:

[0053]

[0054] (2) Single-ended input differential output mode:

[0055] If the single-ended input is the positive input of the driving circuit, disconnect the resistor R15, connect the resistor R16, set R16 = 0Ω, the resistor R16 connects the signal output from U5 to U4, and VIn is pulled down to ground; set R1 = R3, R2 = R4, R5 = R6 = R7 = R8, R13 = R5 + R7, R14 = R6 + R8, R9 = R10 = R11 = R12 = R13 = R14,

[0056] R18=R25=2*R17=2*R26, R20=R21=R23=R24=4*R19=4*R22, we can get

[0057] The output of VIp through U1 is:

[0058]

[0059] The output of V1O through U3 is:

[0060]

[0061] The output of U5 is:

[0062]

[0063] The output VOp of U4 is:

[0064]

[0065] The differential output is:

[0066]

[0067]

[0068] The single-ended input differential output mode can also be selected. When the single-ended input is the reverse input of the driving circuit, the resistor R16 is disconnected and the resistor R15 is connected. R15 is set to 0Ω. The resistor R15 connects the signal output from the operational amplifier U6 to the operational amplifier U3, and VIp is pulled down to ground.

[0069] In summary, through the cooperation of operational amplifiers U1, U2, U3, U4, U5 and U6, differential amplification output can be achieved regardless of single-ended input mode or differential input mode.

[0070] In addition, since the parameters of the resistors R5~R14, R17~R22 of the driving circuit of the present invention can be fixed and no adjustment is required after setting, the basic parameter variables of the driving circuit are only the resistors R1, R2, R3, R4, R15 and R16; therefore, the driving circuit of the present application can be integrated and set as follows Figure 5 The driver module shown has adjustable gain, and external interfaces are reserved for users. That is, the driver module includes a positive input interface VIp, a negative input interface VIn, a bias voltage input interface Vcom, a negative output interface Von, a positive output interface Vop, a power supply interface VCC, a negative power supply interface VEE, a feedback network interface FG, a feedback network interface FB and an MS interface. The MS interface is used to set resistors R15 and R16 when single-ended input is used. The gain parameters of the driver module are adjusted by adjusting resistors R1, R2, R3, R4, R15 and R16.

[0071] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A fully differential ADC driving circuit, characterized in that: Supports single-ended input differential output mode and double-ended input differential output mode, including forward input amplification module, reverse input amplification module, intermediate mode conversion module and signal extraction module. The forward input amplification module is used to configure the differential signal forward input amplification factor, and its two ends are respectively connected to the forward input terminal of the driving circuit and the intermediate mode conversion module, and the forward input terminal of the driving circuit inputs the forward input signal VIp; The reverse input amplification module is used to configure the reverse input amplification factor of the differential signal, and its two ends are respectively connected to the reverse input terminal of the driving circuit and the intermediate mode conversion module, and the reverse input terminal of the driving circuit inputs the reverse input signal VIn; The intermediate mode conversion module is used to realize a single-ended input working mode during signal reverse processing and bias voltage input. It is respectively connected to the forward input amplification module, the reverse input amplification module, the bias voltage input terminal, the reverse output terminal of the driving circuit, and the forward output terminal of the driving circuit. The signal extraction module is respectively connected between the intermediate mode conversion module and the reverse output terminal and the forward output terminal of the driving circuit. The bias voltage input terminal is connected to the reference source Vcom. The signal extraction module is used to extract the reverse input signal in the single-ended mode, and is respectively connected to the intermediate mode conversion module, the reverse output end of the driving circuit and the forward output end of the driving circuit. The reverse output end of the driving circuit outputs the reverse output signal Von, and the forward output end of the driving circuit outputs the forward output signal Vop.

2. A fully differential ADC driving circuit according to claim 1, characterized in that: The forward input amplification module includes an operational amplifier U1, the forward input terminal of the operational amplifier U1 is connected to the forward input terminal of the driving circuit, the reverse input terminal of the operational amplifier U1 is connected to the resistor R1 and then grounded, the output terminal of the operational amplifier U1 is connected to the resistor R5, and a resistor R2 is connected in series between the output terminal and the reverse input terminal of the operational amplifier U1.

3. The fully differential ADC driving circuit according to claim 2, wherein: The reverse input amplification module includes an operational amplifier U2, the positive input terminal of the operational amplifier U2 is connected to the reverse input terminal of the driving circuit, the reverse input terminal of the operational amplifier U2 is connected to the resistor R3 and then grounded, the output terminal of the operational amplifier U2 is connected to the resistor R8, and a resistor R4 is connected in series between the output terminal and the reverse input terminal of the operational amplifier U2.

4. The fully differential ADC driving circuit according to claim 3, wherein: The intermediate mode conversion module includes an operational amplifier U3 and an operational amplifier U4. The positive input terminal of the operational amplifier U3 is connected in parallel with resistors R9 and R11, the other end of resistor R9 is connected to the bias voltage input terminal, and resistor R11 is grounded; the negative input terminal of the operational amplifier U3 is connected to resistor R7, and the other end of resistor R7 is connected to resistor R5; a resistor R13 is connected in series between the output terminal and the negative input terminal of the operational amplifier U3; The positive input terminal of the operational amplifier U4 is connected in parallel with resistors R10 and R12, the other end of resistor R10 is connected to the bias voltage input terminal, and the other end of resistor R12 is grounded; the negative input terminal of the operational amplifier U4 is connected to resistor R8, and the other end of resistor R8 is connected to resistor R6; a resistor R14 is connected in series between the output terminal and the negative input terminal of the operational amplifier U4.

5. The fully differential ADC driving circuit according to claim 4, wherein: The signal extraction module includes an operational amplifier U5 and an operational amplifier U6. The positive input terminal of the operational amplifier U5 is connected in parallel with resistors R17 and R18, the other end of resistor R17 is grounded, and the other end of resistor R18 is connected to the output terminal of the operational amplifier U3; the negative input terminal of the operational amplifier U5 is connected in parallel with resistors R20 and R21, the other end of resistor R20 is connected to the bias voltage input terminal, and the other end of resistor R21 is grounded; the output terminal of the operational amplifier U5 is connected in series with resistor R16 and then connected to resistor R6; and a resistor R19 is connected in series between the negative input terminal and the output terminal of the operational amplifier U5; The positive input terminal of the operational amplifier U6 is connected in parallel with resistors R25 and R26, the other end of resistor R25 is connected to the positive output terminal of the drive circuit, and the other end of resistor R26 is grounded; the negative input terminal of the operational amplifier U6 is connected in parallel with resistors R23 and R24, the other end of resistor R23 is connected to the bias voltage input terminal of the drive circuit, and the other end of resistor R24 is grounded; the output terminal of the operational amplifier U6 is connected in series with resistor R15 and then connected to resistor R5; and resistor R22 is connected in series between the negative input terminal and the output terminal of the operational amplifier U6.

6. The fully differential ADC driving circuit according to claim 5, wherein: When the driving circuit is in a dual-end input differential output mode, the resistors R15 and R16 are in an open circuit state, disconnecting the signals output from the operational amplifiers U5 and U6 to the operational amplifiers U4 and U3.

7. The fully differential ADC driving circuit according to claim 6, wherein: When the driving circuit is in single-ended input and differential output mode, an analog signal is input to the positive input terminal of the driving circuit, resistor R15 is disconnected, resistor R16 is connected, resistor R15 connects the signal output from operational amplifier U5 to operational amplifier U4, resistor R16 = 0Ω, and VIp is grounded.

8. The fully differential ADC driving circuit according to claim 6, wherein: When the driving circuit is in single-ended input differential output mode, an analog signal is input to the reverse input terminal of the driving circuit, resistor R16 is disconnected, resistor R15 is connected, resistor R16 connects the signal output from operational amplifier U6 to operational amplifier U3, resistor R16 = 0Ω, and VIn is grounded.

9. A fully differential ADC driving circuit according to any one of claims 1 to 8, characterized in that: The forward input amplification module, the reverse input amplification module and the intermediate mode conversion module are all connected to a feedback network in parallel. The feedback networks of the forward input amplification module and the reverse input amplification module are used to adjust the parameters of the gain size; while the intermediate mode conversion module does not need to adjust the gain size, and the feedback network and other parameters involved are fixed.

10. A fully differential ADC driving circuit according to any one of claims 1 to 8, characterized in that: The driving circuit can be integrated into a gain-adjustable driving module, which includes a positive input interface VIp, a reverse input interface VIn, a bias voltage input interface Vcom, a reverse output interface Von, a positive output interface Vop, a power supply interface VCC, a negative power supply interface VEE, a feedback network interface FG, a feedback network interface FB and an MS interface. The MS interface is used to set the resistance when single-ended input is used.

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