OPA818: OPA818

Hello Giora,

Would you be able to provide a copy of your TINA circuit files?

Best,

Hasan Babiker

Hasan,

Here it is attached.

Gioraopa818 on off.TSC

Hello Giora, 

I noticed in your picture you were able to get the same response in sim that you are seeing on your board. What input did you use to get this?

Hasan,

In the simulation I used the transient analysis with zero initial value. This imitates going from power off to on.

In the real world we used the power on/off pin

Hello Giora, 

This is a result of both your input diodes and the DC blocking capacitor being used. Take a look at the transient response once these diodes are removed from the circuit:

and now note the response once these diodes are added back in, but capacitor C4 is brought down to 100pF:

Finally you can see below that the initial spike in voltage when the OPA818 is turned on forward biases D2. The rate of change until this diode is turned off is set by capacitor C4 as shown below:

Best,

Hasan Babiker

Hasan,

Thanks.

We tried this but really we are seeing a decay on the output like the decay on VF1.

Giora 

Hasan,

Thanks.

Can not reduce the cap more.

Giora

Hasan,

Attached photos of the settling time of the opa818 Dc coupled no clamping diodes.

Hello Giora,

Can you clarify what the blue signal is measuring? Also how are you measuring the output, ideally this would be measured on a low cap probe with some isolation resistance?

Best,

Hasan Babiker

Hasan,

The blue line is the response of the Power On/Off on the PD pin of OPA818 with a pullup of 2.67K.

See attach drawing.

Hasan,

The blue line is the response of the Power On/Off on the PD pin on the OPA818 with 2.67K pullup.

See attached drawing.

Regards,

Giora

Hello Giora,

The slower settling time here is along the same note and is due to the ac coupling cap.

The initial spike in voltage when the amplifier is turned on will induce a current across C4 & as you can see give some max voltage at VF1. Once the amplifier is working normally and brings both of its inputs to the same potential (in this case 2.4V), VF3 will stay fixed while the voltage across V3 discharges through R3. Since VF3 is still fixed, the output of the amplifier will basically discharge at the same rate because of current across C4 while VF1 discharges  

So some ideas on how to fix this issue:

1. Reduce AC coupling cap which you already mentioned isnt an option here.
2. Switching to a photodiode with higher shunt resistance which probably isn't an option here either (this will limit the voltage at VF1 since current spike should be around the same if capacitor isn't changed).
3. Reducing the trans-impedance gain of your amplifier, so the initial current spikes will have less of an effect on your output.
4. Going back to adding diodes at the input although this will increase your switching time. If going with this option I would recommend using Schottky diodes with a very low forward voltage to reduce switching time as much as possible.

Best,

Hasan Babiker