OPA2140: OPA2140 has abnormal output current when used V/I conversion

Kailyn Chen

Part Number: OPA2140

Hello，

Our customer used OPA2140 as 0~24mA current source, there is no problem during simulation, but actually, he met a question using PCB to test.

1) single 12V power supply, 0~2V input, and output 0~24mA, there is no problem.

2) When +-12V　power supply, there is 0.25~3mA increasement . For example, 1V input, its output is about 10.3mA instead of 10mA. This phenomenon is appeared when 1~2V input. In addition, the output current could only to 18mA and couldn't increase.

Could you please give some suggestions? Or if is there some pcb layout issue? The circuit is as following:

Best regards

Kailyn

over 4 years ago

0 kai klaas69 over 4 years ago

Guru 140200 points

Hi Kaylen,

the circuit isn't stable:

kaylen_opa2140.TSC

Kai

0 Raymond Zhang1 over 4 years ago in reply to kai klaas69

TI__Guru 55491 points

Hi Kaylen,

The Howland Current pump circuit had a Que at approx. 1.5MHz. I compensated the Q and it does not oscillate with small or larger step functions.

Best,

Raymond

0 kai klaas69 over 4 years ago in reply to kai klaas69

Guru 140200 points

Hi Kaylen,

the compensation done by Raymond looks very good.

The 150R resistor, is this the load (burden) of the current pump? Is the 150R burden connected to the current pump via a cable? So, is cable capacitance or other stray capacitance connected in parallel to the burden? This would eventually ask for a modification of compensation.

By the way, why are you using U2 and U1B in the Howland current pump? They add unwanted phase lag into the feedback loops.

Kai

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OPA2140: OPA2140 has abnormal output current when used V/I conversion