Do I really need protection diodes for this amplifier circuit?

[floppy32]

I will make a DAC output amplifier PCB using this quad-opamp(OP484FPZ) and the schematics is below so far:

Brief description about the application:

out1 and out2 will couple to a driver which controls a galvo-mirror. DAC input is coming from a 16-bit DAC(controlled by a STM2 microcontroller board through SPI) it can maximum provide 2.5V to the DAC input of the above circuit. Vp(2.5V) is coming from a precision reference which is the same reference for the DAC as well. The supply voltages of the galvo driver and the above circuit(Vcc and Vee) will be +/-15V from a dual SMPS.

The entire electronics will be inside one of the rack shelf and the other shelf there will be server, computer, other power supplies ect. And the entire application will be indoors.

I don't have experience about opamp protection and I only know some uses zeners or clamp diodes, TVSs with different configurations. When I started to read I was confused what to use at which node in my case and also some even warns diodes can have effect on precision because of their leakage currents.

I just can say the datasheet of the opamp I use doesn't say much about ESD protection and it mentiones:

For input voltages greater than 0.6 V, the input current should be limited to
less than 5 mA to prevent degradation or destruction of the input devices.

I don't want to be super paranoid about it but I wonder if this circuit would require an extra protection. I use all the opmaps of this quad-opamp as you see and I cannot imagine I need to install diodes at all opamp inputs. On the other hand, I cannot make a judgement with my little knowledge if there's such necessity at least for the minimum protection.

 

[betwixt]

If you keep the Vp and DAC signals within acceptable range (usually 0V and supply) you don't need protection diodes. The inputs have low enough impedance to prevent static build up through R7/R8 and through R2/R3 and U3 output.

Brian.

 

[dick_freebird]

For input voltages greater than 0.6 V, the input current should be limited to
less than 5 mA to prevent degradation or destruction of the input devices.

This refers to input -differential- voltage and pertains
to op amps with front end cross-strap diodes. Then
high current in skinny traces "could" be an issue.

Exceeding 0.6V is not going to induce the kind of
drift that an avalanced B-E junction would. The diodes
will take care of all that as long as the current is limited.
In many cases the choice of input / feedback network
components (w/ the source amplitude WC min / max)
will take care of that. Or can be made to.

Applications with "hard step-y" inputs can benefit from
those diodes, or adding Schottkies back-back, in
overdrive-recovery particularly.

 

[KlausST]

Hi,

generally I agree with Brad.
But in detail it depends on your application.

You say "for input voltages greater than 0.6V". This is not true.
It says "differential input voltage" .. which in your case is zero, due to OPAMP feedback.

Sadly the datasheet in the Absolute Maxim Rating, parameter 2 says: "input voltage". Better read it is "common mode input voltage" of +/-18V.

***
You need protection diodes (at input and output) as soon as you expect ESD spikes. Usually when the signals leave the PCB/device using connectors.

***

Hint:
Your circuit doesn´t use any signal filters. This means any high frequency noise will be ssen at the output.
Also usually one doesn not want to see the sharp steps a DAC generates. The output should be smooth.
In detail - again - it depends on your application.
But better add some Cs to the PCB layout - you still may leave them not assembled.


Klaus

 

[floppy32]

Hi,

generally I agree with Brad.
But in detail it depends on your application.

You say "for input voltages greater than 0.6V". This is not true.
It says "differential input voltage" .. which in your case is zero, due to OPAMP feedback.

Sadly the datasheet in the Absolute Maxim Rating, parameter 2 says: "input voltage". Better read it is "common mode input voltage" of +/-18V.

***
You need protection diodes (at input and output) as soon as you expect ESD spikes. Usually when the signals leave the PCB/device using connectors.

***

Hint:
Your circuit doesn´t use any signal filters. This means any high frequency noise will be ssen at the output.
Also usually one doesn not want to see the sharp steps a DAC generates. The output should be smooth.
In detail - again - it depends on your application.
But better add some Cs to the PCB layout - you still may leave them not assembled.


Klaus

Hi thanks for the response,

Who is Brad?

Yes I need RC at the output.

At the input of my circuit opmaps have resistors. But not at the output. So I guess I need ESD protection diodes onlycat the output stage?

So I tried to modify it adding an RC filter for the bipolar output and also Scottky diodes for the bipolar outputs.

I marked the modification in prange square below:

What do you think of this version?

 

[KlausST]

Hi,

Sorry, I meant Brian. Sorry Brian.

As said: it depends on your application.
We don't know what your application is supposed to do.
We don't know what signals you want to generate.
We don't know the expected frequency range.
We don't know whether there are connecors.
We don't know where the signals go to.
... and so on ...

Klaus