[FAQ] INA210: What effect do external resistors on VREF have on CMRR/Gain error?

Problem:

At least once a week customers asks me if they can use a voltage divider to set the reference voltage (VREF) in our devices. They want to do this to reduce footprint and price. Unfortunately, the external resistors will not only impact the accuracy of the REF value, it will also negatively impact the gain error and common mode rejection ratio (CMRR) of the device.

Explanations and Recommendations:

Note: For this explanation I will be using INA21x family as an example.

As with any difference amplifier, the INA21x device common-mode rejection ratio is affected by any impedance present at the REF input. This concern is not a problem when the REF pin is connected directly to most references or power supplies.

If you decide to use a resistive divider off the supply or a reference voltage, we have the following recommendations:

• Buffer the REF pin by an op amp. The buffer isolates the internal impedance from the external impedance of the voltage divider – in other words, the external voltage divider will have no impact on the gain error or common mode rejection if it is buffered. Refer to Figure 1.

Figure 1 Buffering the REF pin

• If you can’t use a buffer, then minimizing the value of the resistor divider resistors (10’s of ohms) will minimize the impact on the additional gain error and common mode error. The downside of the smaller resistor values is the additional current consumption. When selecting the values of the resistors for the voltage divider, the resistor tolerances have to be considered (tolerances from voltage divider and resistors internal to the device).

Another solution would be to process the output as a differential signal into a fully differential ADC. In systems where the INA21x output can be sensed differentially, such as by a differential input analog-to digital converter (ADC) or by using two separate ADC inputs, the effects of external impedance on the REF input can be cancelled. In other words, this would allow the negative effect the voltage divider has on the system to be minimized as the change in the reference voltage would be subtracted out by the differential ADC. Figure 2 shows a method of taking the output from the INA21x family of devices by using the REF pin as a reference.

Figure 2 Sensing the output of INA21x differentially

 How the value and tolerances of the resistors affect the gain error and CMRR?

Figure 3 shows the formulas needed to calculate gain error and CMRR due to the external resistors. Note that these calculations are based on ideal internal resistor values. You would need to take into account the tolerances of internal resistors. Based on simulation (no distribution data or measurements) the temperature variation of the internal resistors is around 25ppm/C and the tolerance is +/-15%. Unfortunately, we cannot commit to a maximum tolerance on the internal components of our device – you cannot think of it as a simple resistor. Our processes say the tolerance is typically ±15%, but since we cannot test it we cannot commit to a number. In order to take into consideration the internal resistors tolerances my suggestion is to replace R2 and R4 resistor values by 15%. Please remember, these are approximations. The results from this do not necessary match device performance.

Figure 3 Formulas to approximate the external resistor effect on gain error and CMRR

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FAQ content by Mayrim Verdejo. For additional questions, please use the e2e forums.
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