VCA824: Fully differential summing amplifier

Hello Mojtaba,

I am working with your TINA schematic and simulation.  Could you confirm your input voltages and desired output voltage levels?

I did find a curve for Av_max = 10V/V where a Vg = 0 setting results in Av = 5 V/V:

You should try Rf = 402 Ohms; the Rf in a VGA product (variable gain amplifier) is not directly setting the signal gain; the Rf and Rg only set the maximum gain.  You can choose to set the maximum gain to 5V/V if you would like, otherwise you can use the Av_max = 10V/V example and set Vg to zero.

I will look into the simulation once you provide your target input and output values.

Best,

Alec

Hello Alec,

Thank you for your reply.

I set the Vg = 1, Rf=200 and Rg=80 to achieve Av = Av_max = 5 V/V. My goal is to design a fully differential summing amplifier, where two inputs are differential signals from twisted pair cables with 100-Ohm impedance. However I am unsure why the result shows an approximate gain of 2.5V/V instead of the expected 5V/V.

I would appreciate any insight you may have.

Hello Mojtaba,

When performing a transient simulation I observe output swing peak-to-peak of 1.44V, which is a gain of ~4.8V/V to your combined 300mVpk-pk input signal.

Do you have a target output voltage value you need to drive the subsequent circuit?

I see you are summing a 50mV sine wave with a 100mV sine wave from your twisted pair cable inputs; this results in a 150mV amplitude sine wave at the VGA.

You should be expecting ~750mV amplitude on the sine wave at the output, with a Vpk-pk = 1.5V.  

While the existing simulation is not exactly what you defined, the output is behaving closer to 5V/V of gain than you originally observed.

Diff-In_Sum-VCA824_TI.TSC

Best,

Alec

Hi Alec,

Thank you for reviewing the simulation file.

I noticed that you changed the Rf value to 402 Ω to achieve a gain of 5 V/V. However, based on the datasheet, the gain should be approximately 2 × (Rf / Rg), which in this case results in 10 V/V. Is there something I might be overlooking?

This gain will eventually be controlled by the output of another stage, but for now, I am struggling to properly set the gain for the summing of two differential inputs.

I would greatly appreciate any insights you may have.

Regards

Hello Alec,

I tried using LMH6554 for adder stage and integrated it with the VCA824 to provide gain control. However, it seems that the VCA824 does not support differential input, so only out+ of the LMH6554 is connected to it, which has a DC offset. when I place a capacitor to block the DC, the total output drops significantly.

Could you please check where I might have made an error?

Regrads

Diff-In_Sum-LMH6554_VCA824.TSC

Hello Mojtaba,

Yes, I can check on this design for you.  Allow me time now and on Monday to investigate.

Best,

Alec