Hi everyone, I need to use an alternative Opamp (TLV342AIDR) as a dark signal replacement for AD8692ARZ for an image sensor board after DAC and I need a 100nF capacitive load allowance for this Opmap, I could not find a distinct outcome about maximum capacitive load allowance in either of the datasheets. Could you please help me in that regard?

Thank you very much in advance
Best regards
Somayeh

 

I could not find a distinct outcome about maximum capacitive load allowance in either of the datasheets. Could you please help me in that regard?

In general, you will need some added circuitry on the output (possibly just a resistor depending upon your signal requirements).
Here's an Analog Devices' discussion of that.

 

Most op-amps will oscillate if a capacitive load is connected.
This paper tells you how to connect a capacitive load without it oscillation
https://www.analog.com/media/en/ana...s-to-avoid-instability-capacitive-loading.pdf
Don’t be fooled into thinking that slow op-amps like the LM324 won’t have a problem. They are the worst.

 

In general, you will need some added circuitry on the output (possibly just a resistor depending upon your signal requirements).
Here's an Analog Devices' discussion of that.

Thank you for your reply,
However, the problrm is that the opamp AD8692ARZ works so far with no problems or extra circuitry, and I just need to replace that opamp with another one that is no longer available with (pin copatible OP) for example TLV342AIDR without changing the layout.

 

Most op-amps will oscillate if a capacitive load is connected.
This paper tells you how to connect a capacitive load without it oscillation
https://www.analog.com/media/en/ana...s-to-avoid-instability-capacitive-loading.pdf
Don’t be fooled into thinking that slow op-amps like the LM324 won’t have a problem. They are the worst.

Thank you for your reply,
However, the problrm is that the opamp AD8692ARZ works so far with no problems or extra circuitry, and I just need to replace that opamp with another one that is no longer available with (pin copatible OP) for example TLV342AIDR without changing the layout.

 

Thank you for your reply,
However, the problrm is that the opamp AD8692ARZ works so far with no problems or extra circuitry, and I just need to replace that opamp with another one that is no longer available with (pin copatible OP) for example TLV342AIDR without changing the layout.

I suggest you follow the advice of the good people from Analog Devices.
No op-amp is guaranteed stable into 100nF.
If you want to continue to push your luck, that’s your choice.

 

I suggest you follow the advice of the good people from Analog Devices.
No op-amp is guaranteed stable into 100nF.
If you want to continue to push your luck, that’s your choice.

I underestand what do you mean, but Im am not a Boss, im not allowed to change the layout. Hier is my task attached!

 

It doesn’t have a capacitive load. There’s 2.2k in series.
That’s a low pass filter at 723Hz, so I’m wondering why anyone specified a 10MHz bandwidth op-amp.
Any old op-amp will do the job, and there won’t be much noise in 723Hz bandwidth.
How important is offset voltage? And rail-to-rail inputs and outputs?
The capacitive load business is irrelevant.

 

It doesn’t have a capacitive load. There’s 2.2k in series.
That’s a low pass filter at 723Hz, so I’m wondering why anyone specified a 10MHz bandwidth op-amp.
Any old op-amp will do the job, and there won’t be much noise in 723Hz bandwidth.
How important is offset voltage? And rail-to-rail inputs and outputs?
The capacitive load business is irrelevant.

Thank you for your reply,

exactly,, I also have the same thinking, I dot Konw why they talk to me about capacitive load capability.

And about offset voltage is so important here. But rail to rail input and output not necessarily!

 

Now I found out that there is an additional resistor between 100nF and the input of the ADC driver, does this also play a role in these senses? See attachment!

 

Now I found out that there is an additional resistor between 100nF and the input of the ADC driver, does this also play a role in these senses? See attachment!

No, it's pointless.

 

No, it's pointless.

Ok, good.
You have asked by the way about how offset Voltage is important, do that play in that regard a role?

 

There doesn't seem to be such a thing as LTC6664. If it did exist, and if it had a lot of input bias current the resistor would cause an offset problem.

 

There doesn't seem to be such a thing as LTC6664. If it did exist, and if it had a lot of input bias current the resistor would cause an offset problem.

you are right. The AD-driver is LTC6404IUD, and the input current of that is 28mA, is that ok?

 

Input bias current = 23uA (typical)
So there will be an offset voltage of 23uA x 499Ω = 11mV

 

Thank you very much for all helps from you!
I discussed with my boss that unfortunately there is no 2.2K resistor, there is a 1 ohm protective resistor. That was my mistake while reading from Altium.
Therefore, there is always the problem of load capability of 100nF.

Kind thanks in advance
Somayeh

 

Increase the 1Ω resistor to 100Ω and everything will be Ok. 1Ω isn’t going to protect anything.

 

Increase the 1Ω resistor to 100Ω and everything will be Ok. 1Ω isn’t going to protect anything.

But I'll miss the bandwidth then, won't I?

 

But I'll miss the bandwidth then, won't I?

So what bandwidth do you need?
What is the ADC's sample frequency?

 

So, what bandwidth do you need?
What is the ADC's sample frequency?

Thank you for your reply.
To be honest with you, I don't know, but the ADC is LTC2203 and the ADC driver before that is LTC6404IUD-1.
They don't give me too much info, they told me Opamp number AD8692ARZ could do this job with 100nF. Now I muss tell them, if Opamp with Mf.p.n TLV342AIDR could do the same job. I have compared already two Opamps, everything except load capability to replace of the camera Board is fine.