AD620 to measure current with shunt resistor

Thread Starter

zfb

Joined Jul 13, 2022
4
My device needs to apply a voltage to a probe, then measure the current which flows through said probe (in the schematic the probe is SLP1).
I've been trying to do this with an AD620, but it wasn't working. I realized that was because both inputs didn't have a path to ground except through the resistor..

Is there a way to make this work with an AD620? If not, can you suggest another method? The currents I will be measuring will be both AC and DC.

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crutschow

Joined Mar 14, 2008
33,369
What is the maximum voltage and current you are wanting to measure?

A high-side current measure IC such as the INA138 may work better for that purpose.

Thread Starter

zfb

Joined Jul 13, 2022
4
There are several places in the device that I was going to try to use them on. The currents will be on the order of +/- 1 mA, so I was going to use a 1.5 k resistor to give about a +/- 1.5 V drop. It can’t be more than 3.3 V because of my microcontroller. And at another part of the circuit it might be as high as 20V (no measured across a resistor, but just measuring the voltage of a floating probe).

Harry Trietley

Joined Jun 29, 2022
10
Crutchow's suggestion - INA138 - sounds good IF your current will always be in the same direction (positive). You say that your current can be DC or AC, +/-1mA. If so, the INA138 output cannot go negative. But then, neither can your AD620 if the negative supply is tied to GND.

Here's an IC I used in a recent design - difference amplifier AD629 or INA117. They have bidirectional voltage outputs if powered from + and -. Their input impedance is much higher - around 400Kohms. They can handle very high common mode voltages: +/-500V (AD629) or +/-200V (INA117), although that probably does not matter in your application.

Note: They have a fixed gain of one, not programmable. That sounds like it would be OK in your application: +/-1.5V at +/-1mA.

OldTech

Joined Jul 24, 2009
7
My device needs to apply a voltage to a probe, then measure the current which flows through said probe (in the schematic the probe is SLP1).
I've been trying to do this with an AD620, but it wasn't working. I realized that was because both inputs didn't have a path to ground except through the resistor..

Is there a way to make this work with an AD620? If not, can you suggest another method? The currents I will be measuring will be both AC and DC.
Your schematic is confusing. Is R21 the probe, or is R21 the current shunt? If it's the current shunt, what does the probe circuit look like? How does the current get into the probe? Is the probe's power source ground referenced?
As your circuit is drawn, there's no path for the probe current to flow. The current needs a complete path, otherwise, it's an open circuit, and no current will flow.

Provide a more accurate and complete circuit diagram, and proper answers will follow.

Cheers,
DaveM

crutschow

Joined Mar 14, 2008
33,369
The currents I will be measuring will be both AC and DC.
What is the nature of the AC signal?

Ian0

Joined Aug 7, 2020
8,975
Watch out for common mode input voltage range. It will not work if the inputs are less than 1.9V above the negative supply or less than 1.2V below the positive supply.The AD620 is definitely intended for dual supplies (positive and negative).
My suggestion would be the TSC2010 or AD8414 (they are very similar to each other). They have a gain of 20, so reduce your current sense resistor by a factor of 20.
They have built-in resistors, so don't need any extra bias resistors (DC path to ground) provided that the IC shares it ground with the rest of the circuit.

Thread Starter

zfb

Joined Jul 13, 2022
4
Your schematic is confusing. Is R21 the probe, or is R21 the current shunt? If it's the current shunt, what does the probe circuit look like? How does the current get into the probe? Is the probe's power source ground referenced?
As your circuit is drawn, there's no path for the probe current to flow. The current needs a complete path, otherwise, it's an open circuit, and no current will flow.

Provide a more accurate and complete circuit diagram, and proper answers will follow.

Cheers,
DaveM
This is a plasma sensor; the current flows from the plasma itself. So even though it looks like an open circuit, once it is introduced to a plasma environment it is not. The schematic is correct. For these purposes, you can assume that current flows freely through magic (unless you are well versed in plasma physics, in which case you already understand it). I need to measure that current, and I am doing so with a resistor (R21). So I simply need an easy way to measure the voltage drop across R21.

Thread Starter

zfb

Joined Jul 13, 2022
4
Crutchow's suggestion - INA138 - sounds good IF your current will always be in the same direction (positive). You say that your current can be DC or AC, +/-1mA. If so, the INA138 output cannot go negative. But then, neither can your AD620 if the negative supply is tied to GND.

Here's an IC I used in a recent design - difference amplifier AD629 or INA117. They have bidirectional voltage outputs if powered from + and -. Their input impedance is much higher - around 400Kohms. They can handle very high common mode voltages: +/-500V (AD629) or +/-200V (INA117), although that probably does not matter in your application.

Note: They have a fixed gain of one, not programmable. That sounds like it would be OK in your application: +/-1.5V at +/-1mA.
This sounds perfect! Thank you so much