Electronic – Opamp: capacitive loading when buffering a Reference

bufferingtrackingvoltage-reference

I need to drive a ratiometric current sensor ACS758xCB with a nominal supply current of 10mA. Due to the ratiometric nature of the sensor I would prefer to supply it from the reference voltage of my ADC. Obviously I need to buffer the reference.

However, most opamps, notably those with low offset voltage, require the capacitive loading of the opamp be limited. Since the sensor itselfs contains active circuitry it requires supply bypassing in the range of 100n.

schematic

simulate this circuit – Schematic created using CircuitLab

How does one usually cope with this situation? I've looked into tracking regulators such as TPS7B4250 but the tracking performance of +/-5mV does not sound too impressive.

Best Answer

This is a common problem having a common solution:

schematic

simulate this circuit – Schematic created using CircuitLab

The circuit provides direct High Frequency feedback from OPA output to its input via C1. DC feedback goes through R2 R3 and is not affected by C1. R1 prevents shortening of HF feedback by the capacitive load. This design can handle 100 uF of load capacitance or even more.

Some comments on components:

  1. You have to consider input offset / bias current of OPA when choosing R2 R3. The voltage drop created by offset current on parallel connection R2 R3 must be smaller than your allowed error.
  2. C1 can be selected experimentally. Too small values cause the circuit to oscillate. Place at least 2 times larger C2 than the largest one that causes oscillations.
  3. R1 can be 10 to 47 Ohms in your case (10 mA load, right?)
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