Hi Sheldon,
The output of VPROPI is not a standard X5 op amp. It is expected to drive a high impedance input and no more than approximately 110uA.
Think of the VPROPI output as a current mirror driving an internal load. At the SENSE pin, the voltage is converted into current using a typical 4.6k resistor. This current then drives an internal load resistor of typically 23k (5X) to create the VPROPI voltage. Adding external resistance to ground creates a parallel resistance path and affects the gain of VPROPI.
To avoid or minimize the effects on the output, there are several options:
1) Add an external amplifer to reduce the voltage into your ADC.
2) Use the voltage on the SENSE pin as the input to your ADC. The full scale voltage at the ADC would be reduced.
3) Change the resistor divider values. Using 3M/2M (or 300k/200k depending on accuracy desired) should reduce the effect of the external load. This could create issues with system noise.
4) Recognize this parallel resistance in the calculation and adjust according. For you, an external load of 15.33k to ground on VPROPI should work.
The gain can be modified by the equation G = (23k||Rext)/4.6k.
The output of VPROPI is not a standard X5 op amp. It is expected to drive a high impedance input and no more than approximately 110uA.
Think of the VPROPI output as a current mirror driving an internal load. At the SENSE pin, the voltage is converted into current using a typical 4.6k resistor. This current then drives an internal load resistor of typically 23k (5X) to create the VPROPI voltage. Adding external resistance to ground creates a parallel resistance path and affects the gain of VPROPI.
To avoid or minimize the effects on the output, there are several options:
1) Add an external amplifer to reduce the voltage into your ADC.
2) Use the voltage on the SENSE pin as the input to your ADC. The full scale voltage at the ADC would be reduced.
3) Change the resistor divider values. Using 3M/2M (or 300k/200k depending on accuracy desired) should reduce the effect of the external load. This could create issues with system noise.
4) Recognize this parallel resistance in the calculation and adjust according. For you, an external load of 15.33k to ground on VPROPI should work.
The gain can be modified by the equation G = (23k||Rext)/4.6k.