# Op amp capacitive loading when buffering a reference

I need to drive a ratiometric current sensor ACS758xCB with a nominal supply current of 10 mA. 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 op amps, notably those with low offset voltage, require the capacitive loading of the op amp be limited. Since the sensor itself contains active circuitry it requires supply bypassing in the range of 100 nF.

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 +/-5 mV does not sound too impressive.

• The datasheet of the Hall sensor says it can operate on a supply voltage between 3 and 5.5 V. So why use 4.5 V ? Most opamps are not so suited to drive 10 mA with only 0.5 V drop (Vcc - Vout) and into a large capacitor. The tracking performance of the TPS7B4250 might not sound impressive but I say that it is enough, now prove that I'm wrong (by calculations etc.). That is how engineers decide if something is OK or not, not by deciding that it does not sound impressive. May 23, 2016 at 8:14
• You don't have to supply the sensor with ref voltage, you supply the sensor with available voltage, like Vcc CPU voltage. Then you have to offset the Vout from sensor with Vref voltage by means of Op amp. The offset should be placed at Vadc/2. May 23, 2016 at 8:19
• Due to the ratiometric nature of the sensor I would prefer to supply it from the reference voltage of my ADC Why ? Provide a proper reason why this is needed. I'd say you cannot provide a good reason because the power supply rejection is not mentioned at all in the datasheet. In my opinion you can feed the sensor directly from +5 V DC. May 23, 2016 at 8:22
• @FakeMoustache: I fully understand that the dropout of 0.5V between the opamp supply of 5V and the sensor supply of 4.5V is marginal for a 10mA load with significant capacitance. That why I ask "[...] How does one usually cope with this situation [...]"
– Arne
May 23, 2016 at 8:26
• @MarkoBuršič My understanding is that for ratiometric sensors the output is a relative to the supply. Now, when my 5V supply has +/-5% over temperature, but my ADC reference has only +/-1.5% over temperature this induces an error. The error would not be there if both sensor and ADC would run on the same voltage.
– Arne
May 23, 2016 at 8:28

## 1 Answer

This is a common problem having a common solution:

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?)
• Is C1 doing the thing called dominant pole compensation here? May 23, 2016 at 12:01