Conditioning signal circuits damaged

Discussion in 'General Electronics Discussion' started by Noise, Jan 27, 2016.

1. Noise

3
1
Jan 27, 2016
Hi everybody,

I'm having an issue with a conditioning circuit that I'm using to increase the output current for some Analog outputs coming from another board. It basically seems to consist in a series of two unity gain buffer realised with two operational Amplifier by Texas Intruments: the first one is an OPA551 and the second one with an OPA548. The problem is that when I apply the input signal without biasing the Operational Amplifiers, I damage them. This doesn't seem to happen when I first bias the amplifers (+15V and -15V) and then I apply the inputs to be amplified.
Unfortunately I have only a pic of the circuit. You can find it in attachment.
Here You can find the datsheets of the two Amplifiers :
http://www.mouser.com/ds/2/405/opa551-404893.pdf
http://www.ti.com/lit/ds/symlink/opa548.pdf

Thanks in advance.

Bye

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2. duke37

5,364
771
Jan 9, 2011
It looks as if the opa551 inputs need to be within 2.5V of the power supply. Thus without a supply they are being damaged with excessive input voltage.

Some proection should be added to stop or limit an input until power is applied.

3. Merlin3189

250
69
Aug 4, 2011
I'd guess your problem is in the Absolute Maximum Ratings,
Input Voltage Range ....................................... (Vā)ā0.5V to (V+) + 0.5V

If V+ and V- are both 0, then this becomes -0.5V to +0.5V, so any input bigger than half a volt exceeds this rating.

Maybe you can have diodes to stop the input going above V+ or below V- ?

But wait for the experts, cuz I'm not one of them!

4. AnalogKid

2,497
718
Jun 10, 2015
What is the input signal voltage range?
What is the input signal output impedance?
What is the input signal current capability?
What is the required output signal voltage range?
What is the output current from each opamp?
How much DC offset error is acceptable?
Is there a reason why the 548 input is coming from the 551 output rather than from the signal input?

ak

5. Noise

3
1
Jan 27, 2016
Thanks to all.

Yes, I Think that that problem is related to maximum range of the voltage input
So what do you suggest to protect the inputs? I don't know if two zener diode could work. Anyway as soon as I can I'll work on it to find a solution. In the meantime every help is welcome
@Ak: Actually I don't know why there are two stage, I didn't project this board. I think that the reason could be related to the possibilty of increasing the the maximum load current, acting on the jumpers. It's only a try...

Last edited by a moderator: Jan 30, 2016
6. Merlin3189

250
69
Aug 4, 2011
I'd have thought a Schottky or Ge diode would be better than a zener, as you need to keep the voltage below 0.5V.
Putting a resistor in series with the input, between the capacitor and the input, might help.

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