Does anyone has the spice model for the AD648 OP-AMP (or similar)
If you have instability at 200 Hz, it's your big loop, and not anything particular w/ the op amp.
You can try a simulation with the ADA4062-2 as suggested, and then try a faster op amp,
the ADA4610-2. Finally, try a much slower bipolar with low IB, the AD8622.
For the integrator stage, the simulator may have problems finding a DC bias point, so you may
have to use a .IC spice directive on the integrator output set to zero volts. You can also, in the sim,
and in real life, add a large resistor (1meg?) across the 1uF cap to form a "leaky integrator" or a low
pass filter. I don't know if this would be acceptable in your system or not. Finally, If you are running
on +/-15V and the integrator is integrating up and down, and the output gets to +10V or -10V, then the
cap is charged up. If you turn off the system and the power supplies drop to zero rapidly, the cap
will discharge backwards into the output and destroy the part. Maybe one time out of a thousand,
but I did have a customer with this type of failure several years ago. The good news is the 1.69k
resistor will limit the current so you might not have a problem.
A problem that has been there since day one is the heavy load on the output, you have less than
1k. (R4/R4A) The AD648 has a wimpy output stage; the spec table has output swing w/ a 10k load.
Your output is much lower; see the graph.
Q) What's on the other side of D7???
We totally rewrote the photodiode tool last year and it's very good; so if you have the specs
for your photodiode, try it out:
PHOTODIODE CIRCUIT DESIGN WIZARD | Analog Devices
Another good site for TIAs is: JavaScience Consulting Services
He has some good calculators there.
You can use ADA4062-2 as a recommended alternative parts. It's a low power JFET dual op amp with a readily available SPICE model.
Please let us know if you have any more question.
I am analyzing an old design and I need to make some changes from here 1st.
I still need to get a copy of the SPICE model (or similar)
I understand your concern. This device is an old part and I'm not sure if there is a spice model still available for distribution. I'll let you know if I get one.
can you suggest a similar spice model that i can use in my simulation?
The signal we are looking is below the range of 1HZ but we are seen instability in the 200HZ band
would like to have the model that includes the poles & zeros so I can check for stability
Thank you for your help
Can you attach a partial schematic?
please find a section of the schematic
The feedback loop is optical and it is not included in this sch
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