Get opamp output voltage to zero voltage

opamp output voltage to zero volt

Hi,

I have applied zero volt to a opamp buffer. The output is 1.3mV. I think its because of the opamp offset voltage. (tested with ADA4522, LTC1151, LTC2057).

Since I'm designing a variable current source of 0 to 25mA. Because of 1.3mV, it causes a flow of current of 7uA with 200 Ohms resistor. How can I achieve it to 0 volt?

Re: opamp output voltage to zero volt

Op amp offset null option can be used to reduce the offset which you are getting.

Re: opamp output voltage to zero volt

Hi,

Please learn to give enough information.
--> Schematic, Power supply informations, test conditions.

I assume - because you don´t give enough information - that you used single supply and thus you have a problem with output voltage close to supply rails.

--> use a dual supply.

Klaus

Re: opamp output voltage to zero volt

Considering that ADA4522 is a "zero drift" (chopper stabilized) OP with specified maximum offset voltage of only 5 µV, it's quite obvious that you have other offset sources than OP offset voltage in your circuit.

I fear the question is useless without an exact circuit schematic.

Re: opamp output voltage to zero volt

Without the schematic or additional information, I have the same assumption as Klaus: He is using a single supply to power the opamp.

Re: opamp output voltage to zero volt

To measure current I tried to use a 1458 dual op amp. Single supply. It did not give me 0V output from 0V input. I went through all kinds of contortions in an effort to solve it. I ended up making a 3-resistor network. I adjusted their values to apply just the right voltage to the input. It was cumbersome.

I could have saved myself the trouble by using a single 741 IC because it has two terminals for offset adjust. I only used a dual IC package because my circuit had two op amps and I thought to do it the 'economical' way. Instead I should have used two single op amp IC's.

Re: opamp output voltage to zero volt

Really sorry guys for not providing enough information. Since I am testing on breadboard, I haven't drawn any schematic using software. Just a sketch is attached. This is my circuit.

I have connected a keithley multimeter in series with the load and measuring the current.

First I tried with LTC1655 which has internal reference, loaded DAC value as 0, the measured current is 0.005 mA
Then I tried with LTC2641, the measured current is 0.004 mA.

Is it possible to achieve 0.000mA?

Resistor used is Vishay 0.01% tolerance.

I think I tried to explain it completely what is my problem.

Re: opamp output voltage to zero volt

LTC1655 datasheet says zero scale output error up to 3 mV!

Still lack of information, see below

Re: opamp output voltage to zero volt

Hi,

Still don´t know if you are using single supply...and where exactely you measure the voltage.

***
But you need to understand that 0V in a signal supply is a physical limit.
And with any output current you will never get zero output voltage.
You may come close to it but not exactely.

Therefore I recommend to build your circuit so that there is a headroom to both limits.

You could design your circuit with amplifier "B" to output 0mA with an input voltage of 10mV (or any other useful positive voltage)
(two resistors: one 1M00 and one of about 5k. (the exact value of 4882.8Ohms is not necessary, just tell the software the correct value)
DAC_VRef --> 1M00 --> -IN --> 5k --> existing 200R shunt.
Then you need a digital DAC input of 161 (decimal) to produce just 0mA of output current.

Calibration for offset and gain in software is easy: just a simple ADD and MULTIPLY.

But it makes life more easy.

Klaus

Re: opamp output voltage to zero volt

That very last opamp.....are you operating it with a single supply? Why?

No opamp can swing its output to 0 volts (as referenced from its VSS or VEE supply).
Rail to rail devices manage less than 100mV, but still, it is not zero.