# Current-to-Voltage Op Amp Circuitry

Discussion in 'General Electronics Chat' started by Candella, Jul 1, 2014.

1. ### Candella Thread Starter New Member

Jun 18, 2014
2
0
So what I'm trying to do is build a current-to-voltage converter for the purpose of measuring the input alternating voltage signal and output current of a dielectric sample in series with an op-amp. The circuit can be found here at page 11. http://interactive.npl.co.uk/multiferroics/images/7/7a/CMMT_A(152).pdf

Just a note that I don't have much electronics background, very limited in fact, and what you see here is my attempt at replicating a circuit.

For this circuitry, I will be applying an AC voltage of 20Vpp to 100Vpp at a maximum frequency of 200kHz. The sample I will be using is a Piezoelectric material with about 400pF capacitance. My voltage supply will be from a signal generator connected to an amplifier whose output is a BNC. And for recording the signals, I will be using passive scope probes.

The questions I have are:
1. What op-amps are suitable for what I am trying to achieve? It is stated that a 741 op-amp is suitable. What are better alternatives?

2. Am I right in saying that input power supply for the op-amp is unnecessary?

3. The drawing shows a virtual ground but how would the circuitry be wired up? I am guessing that the ground output (shielding) from the BNC connector connect to both the '+' and 'output' terminals on the op-amp?
And the +/- output from the BNC connect to the ' - ' terminal on the op-amp... correct?

4. Since I will be using passive scope probes. At which points in the circuitry do I connect the probe for the Vout signal? Would it be the output terminal of the op-amp?

2. ### wayneh Expert

Sep 9, 2010
12,403
3,249
Any and every op-amp would be better than the 741. But you'll need a fast op-amp for working at 200kHz - that will limit your choices. You'll need to divide down those input voltages to fall within the power supply voltages you supply the op-amp, or else you will damage your op-amp.

All op-amps require a power supply. It's omitted for clarity in Figure 6. Were you thinking you could power it from the inputs alone? You cannot. But you might be able to condition the power so that you can power the op-amp from it.

3. ### wayneh Expert

Sep 9, 2010
12,403
3,249
Here's the OP's circuit:

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4. ### Candella Thread Starter New Member

Jun 18, 2014
2
0
Hi Wayneh,

thanks for your help and the attachment.

1. Regarding the choice of the op-amp what specifications do I have to consider for choosing a "fast" op-amp?

2. Could you elaborate more on your second point about dividing the input voltages?

5. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
We can't make you an expert in opamps in a couple of posts, but with further research you can start with the following:
1. You can start with GBW (Gain-Bandwidth Product). This number tells you over what bandwidth the opamp has a potential gain of greater than 1. If GBW = 12 kHz it means you can get a gain of 12 at 1 kHz or a gain of 1 at 12 kHz.
2. You can look at Slew Rate. This is expressed in Volts per microsecond and is an indication of how fast the output can move in response to a change in the input. A larger Slew Rate implies a faster part.

A little bit of speed is a good thing. A lot of speed is a sure recipe for designing an amplifier and ending up with an oscillator. This is usually a bad bad thing.

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