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Part Number: INA210
I want a simple, cheap solution to provide a reference to the VREF pin of my device. Are there any issues with using a voltage divider to set VREF?
Patrick Simmons, TI Sensing Products Applications Support
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TI makes no warranties and assumes no liability for applications assistance or customer product design. You are fully responsible for all design decisions and engineering with regard to your products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning your designs.
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:
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.
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.
FAQ content by Mayrim Verdejo. For additional questions, please use the e2e forums.TI makes no warranties and assumes no liability for applications assistance or customer product design. You are fully responsible for all design decisions and engineering with regard to your products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning your designs.
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