Author Topic: A quick question on differential op amps  (Read 1237 times)

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Offline EteslaTopic starter

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A quick question on differential op amps
« on: February 09, 2019, 05:39:14 am »
I have a question about differential amplifiers. Say I have an op amp in the standard differential op amp configuration (like this: https://www.electronics-tutorials.ws/opamp/opamp_5.html), where all the resistors are say, 10K (gain of -1). I power the op amp with +/- 15 volts. How far outside of that range of +/- 15V am I allowed to go on the inputs of the circuit (V1 and V2 in the webpage example)? Can I measure the difference between 100 and 101 volts, or will this hurt the amplifier? I'm not asking about specialty devices here, just a 'standard' op amp like the 741 or something like that.[/size][/size] If this is not allowed, what parameters in the data sheet should I be looking at to determine how far out of the range I can go? Thanks in advance!
 

Offline awallin

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Re: A quick question on differential op amps
« Reply #1 on: February 09, 2019, 07:19:12 am »
unless it's a special op-amp you can't go much outside the supply rails V-/V+ on the +IN and -IN pins.
One approach is an attenuating voltage divider in front of the inputs - if you need to measure 100V.

some purpose-made voltage/current-sensing amplifier might have an 'isolated' input stage and be capable of handling common-mode voltage beyond the DC-rails on the input pins... but not a standard op-amp AFAIK.
 

Offline not1xor1

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Re: A quick question on differential op amps
« Reply #2 on: February 09, 2019, 08:31:03 am »
I have a question about differential amplifiers. Say I have an op amp in the standard differential op amp configuration (like this: https://www.electronics-tutorials.ws/opamp/opamp_5.html), where all the resistors are say, 10K (gain of -1). I power the op amp with +/- 15 volts. How far outside of that range of +/- 15V am I allowed to go on the inputs of the circuit (V1 and V2 in the webpage example)? Can I measure the difference between 100 and 101 volts, or will this hurt the amplifier? I'm not asking about specialty devices here, just a 'standard' op amp like the 741 or something like that.[/size][/size] If this is not allowed, what parameters in the data sheet should I be looking at to determine how far out of the range I can go? Thanks in advance!

If you have a rail-to-rail input opamp you can't exceed +/-30V as the resistors would divide the input voltage by 2.
If you need higher voltages search "high common mode voltage differential amplifier". Those usually have a 5th resistor to reduce the voltage at the opamp inputs.
 

Offline rstofer

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Re: A quick question on differential op amps
« Reply #3 on: February 09, 2019, 11:01:06 pm »
Pick an op amp and read the datasheet.  We talked about this last week and the op amp chosen for discussion could handle 30+ volts.

https://www.eevblog.com/forum/beginners/will-an-op-amp-be-damaged-if-input-voltage-is-higher-than-supply/

The uA741 can not handle much in the way of overvoltage.  The way I read it, the maximum input voltage is +-15V with a maximum Vcc of +-18V.

The maximum differential voltage is also +-15V

http://www.ti.com/lit/ds/symlink/ua741.pdf  Page 5

There are specialty op amps but at very high voltages, they are going to be scarce.
« Last Edit: February 09, 2019, 11:03:37 pm by rstofer »
 

Online David Hess

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Re: A quick question on differential op amps
« Reply #4 on: February 09, 2019, 11:04:10 pm »
I have a question about differential amplifiers. Say I have an op amp in the standard differential op amp configuration (like this: https://www.electronics-tutorials.ws/opamp/opamp_5.html), where all the resistors are say, 10K (gain of -1).

That really should be called an instrumentation or difference amplifier because differential amplifier has an existing and very different meaning which includes differential outputs.

Quote
I power the op amp with +/- 15 volts. How far outside of that range of +/- 15V am I allowed to go on the inputs of the circuit (V1 and V2 in the webpage example)?

There are two requirements:

1. The input common mode range of the operational amplifier may not be exceeded.
2. The output must not saturate.

The divider ratios set the gain but the R2/(R2+R4) divider sets the common mode input voltage so at lower gains and especially gains below 1, the input common mode range increases above the supply voltage.  A 741 running on +/-15 volts could have an input common mode range of +/-10 volts but operating as a 4 resistor instrumentation amplifier with a gain of 1/10, has an input common mode range of +/-100 volts.  At a gain of 10, the input common mode range is more like 10 volts.
 

Online Zero999

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Re: A quick question on differential op amps
« Reply #5 on: February 10, 2019, 12:07:21 am »
What are you using the differential amplifier for?
 

Offline alex.martinez

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Re: A quick question on differential op amps
« Reply #6 on: February 10, 2019, 07:39:38 pm »
Take into account that the input of the opamp is a differential pair. They have an associated maximum input depending on the model, and you can find it under the common mode voltage range input. It usually goes from values of 0 to Vcc - x for the opamp to remain stable.

I would guess that if you want to obtain the subtraction of the signal you mention, a good approach is to use a voltage divider resistor config, to scale the voltage down by a known factor.
 


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