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I am working on an electronics project to design a transimpedance amplifier circuit. It involves an op amp chip in a PCB with a current input, 3.4 KOhm resistor, and the voltage output that I am measuring.

  • Why might it be that for nearly 20 of the same OPA227P chips, the voltage offset is about .5V? I checked the chip's datasheet and its offset should be 75 uV maximum. The chip is in the PCB, no input, powered by a 12 V source, grounded to a metal table... how can this problem be fixed?
  • And, why is the voltage output that I see not stabilizing on a single value? It fluctuates up and down a lot and is generally a lot lower than expected, the gain is lesser on a magnitude of 10-100. Is there something that I can do so that the voltage output stabilizes? Why might the gain be so lower than expected, is there something I could do to remedy that?

Thank you for any help!

More details:

Here is a diagram of the circuit: enter image description here

The chip is being powered by a 12 V source (when I measure its voltage with a multimeter, it's actually 11.84 V). The current input is from a programmable current source. I've been using .1-1.0 mA in .1 mA increments. I've also been checking the current input with a multimeter. The op amp chip model is OPA227P, and it's in a socket which is soldered onto the PCB. I measured the resistance of the resistor, it is 3.398 KOhm. The other leads of the current source are grounded and the ground port of the PCB is grounded, all to a metal table in the lab but on different spots (there is no measurable voltage difference between those points).

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    \$\begingroup\$ Hi, there isn't sufficient information to answer your question. Make sure when writing that you provide all necessary details to a person unfamiliar with your circuit. Be specific. Instead of "transimpedance amplifier circuit" provide a diagram of the circuit (with the tool). \$\endgroup\$ – Voltage Spike♦ Jan 24 '19 at 5:55
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    \$\begingroup\$ Please share the schematic of your circuit, including power supplies, if you want to get a useful answer. \$\endgroup\$ – The Photon Jan 24 '19 at 6:14
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    \$\begingroup\$ If you want a useful answer from us, you MUST provide us with a schematic or a diagram with the issues you mentioned. \$\endgroup\$ – VTNCaGNtdDVNalUy Jan 24 '19 at 6:17
  • \$\begingroup\$ I'm sorry about that. I added more details by editing the question. Thank you for letting me know! \$\endgroup\$ – Jessica Jan 25 '19 at 20:01
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Why might it be that for nearly 20 of the same OPA227P chips, the voltage offset is about .5V? I checked the chip's datasheet and its offset should be 75 uV maximum. The chip is in the PCB, no input, powered by a 12 V source, grounded to a metal table... how can this problem be fixed?

First off... A floating input is not a good way to test a circuit. If you want to check the input, then ground it. This is how texas instruments tests for voltage offset. (by switching s1 and s2 on as shown below)

enter image description here Source: www.ti.com/lit/an/sloa059/sloa059.pdf

And, why is the voltage output that I see not stabilizing on a single value? It fluctuates up and down a lot and is generally a lot lower than expected, the gain is lesser on a magnitude of 10-100. Is there something that I can do so that the voltage output stabilizes? Why might the gain be so lower than expected, is there something I could do to remedy that?

Because the input is floating! If you look at the output, it's most likely outputting a sine wave at 60Hz. The input bias current is 1nA for this part, with nothing driving the input this amplifier could pick up a lot of noise from many different sources. (if your input isn't floating, then you probably haven't shielded your current source very well from noise)

If it's not 60Hz the amplifier is picking up and you only have a gain resistor then it may be that the amplifier isn't compensated. You need some capacitance to compensate the amplifier.

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  • \$\begingroup\$ It's not just the fact of the noise. Depending on the layout of the input, it's possible that the op amp input stage is being overdriven, which will produce really weird results. \$\endgroup\$ – WhatRoughBeast Jan 24 '19 at 7:22
  • \$\begingroup\$ Hence the compensation comment, with a gain of 100 and a an input bias current of 1nA it is unlikely that parasitics are coming into play \$\endgroup\$ – Voltage Spike♦ Jan 24 '19 at 7:31
  • \$\begingroup\$ Sorry, but that's not what I'm talking about. RF is typically beyond the frequency range of an op amp (except the really fast ones), and no compensation will allow dealing with such inputs. And by layout I'm not talking parasitics in the sense of modifying frequency response. I'm talking about pickup and antenna effects. \$\endgroup\$ – WhatRoughBeast Jan 24 '19 at 14:00
  • \$\begingroup\$ @WhatRoughBeast If the opamp rectifies rf then yes. \$\endgroup\$ – Voltage Spike♦ Jan 24 '19 at 15:49
  • \$\begingroup\$ The input is made up of semiconductor junctions which are by their nature rectifiers. \$\endgroup\$ – WhatRoughBeast Jan 24 '19 at 22:37
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It's possible your op-amp is oscillating, especially if the input line is long, but open. Less than 10pF of stray capacitance could be enough.

If you have an oscilloscope, look at the op-amp output.

You can try connecting a capacitor across the feedback resistor, try something like 100pF to start with. Of course that will limit the bandwidth.

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  • \$\begingroup\$ Thank you for the insight! I will look at the op-amp output with an oscilloscope and try connecting a capacitor across the feedback resistor. \$\endgroup\$ – Jessica Jan 29 '19 at 4:14

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