# Op-amp with constant gain for small signal(mV)

simulate this circuit – Schematic created using CircuitLab

I want to measure current using shunt resistor and I want to amplify the voltage over the shunt using an op-amp. The shunt resistor gives 60mV @ 60A. I tried LTC 1050(http://cds.linear.com/docs/en/datasheet/1050fb.pdf) but for input <20mV the gain is not constant. The op-amp has a supply of +/5V. Vout is the signal that I want to send to ADC NI 9215, which has ±10 V measurement range and 16-bit resolution. Do you know what features the op-amp should have for this purpose? Which op-amp do you suggest to use? Which circuit is the best for this application?

Thanks

• There's no reason the gain would vary depending on just the input level. Post your circuit and explain what makes you think the gain is changing.
– JRE
Mar 31 '17 at 10:39
• Show the circuit you tried. Mar 31 '17 at 10:39
• for input <20mV the gain is not constant I doubt that as this is highly unlikely assuming a proper design. Explain how you have come to this conclusion Mar 31 '17 at 11:01
• I added the circuit and more details in the description.
– Anto
Mar 31 '17 at 11:50
• No, show the complete circuit including the power rails and the load that takes 60 amps. Mar 31 '17 at 12:16

Do you know what features should have the op amp for this purpose?

for that particular circuit, you need dual supply (or a beyond-the-rail opamp), zero offset opamp. it is not a good circuit to measure current.

• Maxim have some pretty decent current sensing amplifiers
– MCG
Mar 31 '17 at 12:42
• Which is a good circuit to measure current?
– Anto
Mar 31 '17 at 12:46

You need a opamp that includes ground in its common mode input range. You didn't provide a link, so we don't know if the one you are using has this spec.

However, even for opamps that supposedly can handle input signals down to ground, it can often be a good idea to get them out of that region by providing a little negative supply. Just a charge pump running from something that toggles often is good enough. Sometimes it's easy to arrange for a microcontroller to put its clock on a spare output pin, for example.

## Update

You have now changed the description of your circuit to this schematic:

This is very unlikely to work. A TL081 is inappropriate here. Note that with ±15 V supply, its common mode input range is only ±11 V and its output voltage swing ±12 V with 10 kΩ load. Put another way, it needs 4 V input headroom from each supply, and 3 V output headroom. The datasheet doesn't say how this might scale to only a ±5 V supply, but probably not well at all. This amp really isn't intended for only a ±5 V supply.

There are plenty of opamps that would work fine in this application with ±5 V supply, but the TL081 is not one of them. You need to actually READ THE DATASHEET!

• this is the op amp cds.linear.com/docs/en/datasheet/1050fb.pdf
– Anto
Mar 31 '17 at 12:19
• @Anto Next time, type the opamp model in the schematic. You now see that it is assumed you used a TL081 which is unsuitable for this. But you used a different opamp. Then why draw a schematic where it says TL081 ? Details are important ! Mar 31 '17 at 13:01
• @Olin Lathrop do you think that if I substitute my op amp with TL081 the circuit will work correctly?
– Anto
Mar 31 '17 at 13:11
• > do you think that if I substitute my op amp with TL081 the circuit will work correctly? the circuit, as is, should work. unfortunately, you have done a great job at preventing others from help you. Mar 31 '17 at 16:05