OPA551: OPA551

Rajesh Kumar72

Part Number: OPA551

Hello,

I am using OPa551UA in design as inverting amplifier with gain of 2.

Please see the below circuit:

The above circuit is realized on the board and voltage measured at the output of the OPA551 is -5V, but it is designed for -10V. Also voltage divider o/p is measured as 2.5V instead of expected 5V.

When I simulated the same circuit using PSpice, results are matching with the measured values.

Is it loading of of voltage divider circuit due to gain resistor network, R3 and R4. After removing R3, I get 5V at the o/p of divider network.

What is the output impedance of OPA551UA, not specified in the datasheet?

Could you comment on the behaviour of the above circuit. Why the opamp output is not -10V as expected?

Thanks & Regards,

Rajesh Kumar

over 4 years ago

0 Raymond Zhang1 over 4 years ago

TI__Mastermind 48006 points

Hi Rajesh,

Question:

Is it loading of of voltage divider circuit due to gain resistor network, R3 and R4. After removing R3, I get 5V at the o/p of divider network.

Answer:

The V+ and V- of OPA551 is at virtual ground. The V- voltage is at 10V*(R2||R3)/(R1+ R2||R3) = 10*0.25=2.5V. This is the reason why you are getting -5V at Vout.

If you want to use the circuit, you may use on of the following options.

1. Place a buffer after the voltage divider and in front of R3 resistor.

2. Make R2<<R3, so R2||R3 is approx. equal to R2. You will have some voltage errors in the configuration.

3. If you use the op amp to obtain -10V voltage rail, you may perform a voltage divider from -15V rail, or generate -10V from negative LDO or use zener to do so.

Best.

Raymond

0 Rajesh Kumar72 over 4 years ago in reply to Raymond Zhang1

Intellectual 500 points

Thanks Raymond for quick clarifications.

True, virtual ground concept got clarified through this circuit.

Could you please tell me about terminating the V+ terminal through R5 resistor. Is R5 optional or mandatory? What benefit we get by using R5 and how do we select a suitable value for our application?

Regards,

Rajesh Kumar

0 Raymond Zhang1 over 4 years ago in reply to Rajesh Kumar72

TI__Mastermind 48006 points

Hi Rajesh

The placement of R5 can be used to compensate Vos and Ib sometimes due to imbalanced of mirror current in the first stage of an Op Amp, see the attached video link.

https://training.ti.com/ti-precision-labs-op-amps-vos-and-ib-lab?context=1139747-1139745-14685-1138797-14684

In general, placement of R5 resistor is not necessary. R5 is calculated from R3||R6 or ~2.27kOhm. Since the circui has gain of 2 (Op Amp's noise gain = 3, since noise gain is defined as injected Vos at V+ pin.) the output voltage due to Vos and Ib in contributed errors can be negligible, see the attached Tina simulation.

OPA551 E2E 03022020.TSC

OPA551 E2E R5 03022020.TSC

Best,

Raymond

0 Rajesh Kumar72 over 4 years ago in reply to Raymond Zhang1

Intellectual 500 points

Hi Raymond,

Thanks for the clarifications.

Regards,

Rajesh

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OPA551: OPA551