# Choosing opamp with sufficient output current for Voltage Reference with high impedance load

I am designing a differential sensor that will be placed on the skin to detect biopotentials. Two inputs are fed into an instrumentation amplifier that has a reference set at 1.5 volts (VREF15 in the schematic). That 1.5 voltage reference is also tied to the skin through a "ground" electrode. The 1.5v reference is used as "ground" in order to capture the full signal wave as the sensor is powered by a single battery.

Using the LTC2055 as a buffer, I've found that I'm not getting enough output current to drive my reference. Is the high impedance of the skin the problem (skin is "SigC" in the schematic)? What do I need to do to calculate the necessary output current needed for the opamp?

The short circuit output current of the opamp I'm using is 140 micro amps.

• You need to add resistance to guarantee less than 50 micro amps to the body through the ground Aug 21 '15 at 15:30
• I'm slightly confused. Is the minimum voltage for this part not 3V. And do make sure no voltage gets to the skin otherwise you have a case of hydrolysis. Aug 21 '15 at 17:18
• U8A/B are both powered by 3.3V (AVDD). Never heard of a reference electrode causing hydrolysis in a patient - the voltage sets the body's "Ground" 1.5V above the battery's ground so that you can get bidirectional waveforms with a single supply Aug 22 '15 at 3:20

## 1 Answer

The short-circuit current of the LTC2055 is actually several mA. Check the datasheet. It's micropower, but not that much.

The current is limited to Avdd/78K + 1.5V/100K by the 78K resistance between the op-amp and the electrode. From that I conclude that your Avdd must be about 10V.

You can reduce the resistances R15-R17 and increase the current. I have no idea whether that is safe from a biological point of view, but halving each of them would double the current.

• Thanks for the input! My Avdd is 3.3V, currently (heh) the simulation says the current on SigC is 136pA, so well within safety params if I cut the resistors by 10x. Aug 21 '15 at 15:40
• Please read @ScottSeidman's comments on safety. I think he has a lot of relevant experience in this area. Aug 21 '15 at 15:43