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| A quick question on differential op amps |
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| Etesla:
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! |
| awallin:
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. |
| not1xor1:
--- Quote from: Etesla 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! --- End quote --- 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. |
| rstofer:
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. |
| David Hess:
--- Quote from: Etesla 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). --- End quote --- 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)? --- End quote --- 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. |
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