Cascading OPA847 and VCA821

Paul Law49

Intellectual 260 points

Other Parts Discussed in Thread: VCA821, OPA847

I cascaded the OPA847 and VCA821

my input signal was around 20mV (sine wave)

the OPA847 was set at a gain of 20.

the VCA821 was set at a gain of 2.

However, the output at the VCA821 looked like a square wave. What seems to be the problem here? How can I make the final output to be a sine wave?

over 7 years ago

0 Samir Cherian over 7 years ago

TI__Genius 12631 points

Paul,
I cannot think of anything off the top of my head.
1. Can you please attach a schematic.
2. Have you probed the OPA847 output to see if you are get a 400mVpp sine wave?
3. Have you tested your circuit in TINA?

0 Paul Law49 over 7 years ago in reply to Samir Cherian

Intellectual 260 points

0 Samir Cherian over 7 years ago in reply to Paul Law49

TI__Genius 12631 points

What package are you using for each one? I don't see the pin numbers. On the VCA821 if the numbers 7 and 9 refer to pin numbers then something is wrong. Pin 7 is Vin- on the VSSOP package and VREF of the SOIC package. Please simulate the circuit in TINA just as you have designed it on the PCB and see if it works.

Samir

0 Paul Law49 over 7 years ago in reply to Samir Cherian

Intellectual 260 points

cascade.TSCin the simulation i used the vssop package of vca821 since i can't find the soic package of it. In my hardware implementation i used the SOIC package of VCA821.

0 Samir Cherian over 7 years ago in reply to Paul Law49

TI__Genius 12631 points

According to the datasheet there should be a 20ohm resistor from Vref to GND. I believe that only improves the amplifiers stability.
Do you think the part is oscillating? Does the output frequency change when you change the input frequency?

If you don't apply a sinusoidal signal to the OPA847 what is the dc output voltage of the OPA847 and the VCA821?

If none of these work then you may have an error in your schematic or your test setup. What is the quiescent current consumed from the +/-5V supplies with no sinusoidal signal on the inputs?

Amplifiers

Amplifiers forum

Cascading OPA847 and VCA821