OPA192: Howland Current Pump Circuit with OPA192

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Onedrive link to see schematic

Hi John,

I think there was something corrupted in your TINA file as I was having simulation problems. I copied the circuit into a fresh TINA restart and then things began to simulate as expected. You will find the corrected TINA file attached.

Since the Improved Howland Current Pump provides a constant output current for a constant input voltage he OPA192 output voltage moves to a specific level in proportion to the load resistance. At some higher load resistance level the output voltage rises to the point where the OPA192 output voltage has no additional range remaining. The load resistance can be increased no further if the circuit is to remain operating in a linear manner.

Below I show the OPA192 Improved Howland Current Pump with a load resistance range of 0 Ohms to 5 kilohms. Somewhere beyond a load resistance resistance of 5 kilohms the output will reach its maximum swing to the supply rail. Increasing the maximum resistance in the TINA DC transfer analysis would reveal where that occurs, or it can be easily calaulated.

Regards, Thomas

Precision Amplifiers Applications Engineering

OPA192_IHCP_01.TSC

In practice I am having problems with a 2K load resistor.

It is stopping at a command voltage of 2.5 Volts.

If I go over that is seems to shut down.  That is my issue. I agree with your reply above. But it should work at 5.00 volts with 2K load.

Hi John,

I am not having an issues with it simulating when the input voltage exceeds 2.5 V. See the image from the simulation shown below. This is using the TINA file I posted.

Regards, Thomas

Precision Amplifiers Applications Engineering

It does work in practice with 5 Volt command with a 100ohm load.

But not a 2K load.

Any ideas on what to look at?

Hi John,

The OPA192 and most higher-voltage op amps should work in your Improved Howland Current Pump circuit when the circuit is functioning correctly. The real circuit performances should closely mimic what you receive from simulations. Since you are running into an issue when Vin is higher than 2.5 V, then something about the circuit, or a component, is not correct. 

Here are some basic things to check:

1. Recheck your circuit wiring.
2. Check the resistor values with a DMM to make sure they match the schematic values.
3. Confirm the input voltage and supply voltage levels with a DMM.
4. Check the OPA192 output for any signs of oscillation using a DSO and 10x probe.
5. Replace the OPA192 with a fresh device, or another high-voltage op amp. See if the circuit then functions as expected with a  >2.5 V input.
6. A issue we run into with some customer circuits is PC board cleanliness, or the lack of. Residual solder flux on the board can support board leakage currents. All PCBs should be properly cleaned before use.

Let me know what you find.

Regards, Thomas

Precision Amplifiers Applications Engineering