PGA855: PGA855 Simulated data

Hey Joseph, 

The error that you are seeing is due to the supply voltage of the output being set too low. 
The recommended operating conditions state that LVDD should be at least 4.5V in single supply.

I also recommend raising your VOCM to 2V to achieve the full output swing.

The simulation model is helpful in evaluating design performance but does not cover input and output range. We have the following calculator tool on the ti.com product page to help with that: 

Since the expected input is -15V to 15V, differential of 30V, a single supply is insufficient: 

A dual supply on the output allows this: 

At a diff voltage of 30V, the expected output is then 0.0688 to 3.9312. 

Section 9.1.1 of the datasheet covers this in a bit more detail. 

Do you have another power supply rail that can be used? 

All the best,
Carolina 

Thank you very much for your reply.

Thank you very much for pointing me to the calculator this was very helpful

Good spot on the output supply voltage. Yes, I do have a +5V supply I can use but I want the output to be a single supply (I want to keep LVSS at GND).

I have to keep the VOCM at 1.5V in my design. Therefore, I know that this gives me a maximum input of 17.6V differential voltage that I can measure and that's fine in my design.

I was not clear when I specified the simulation in the previous post, I always keep INN at 0V.

On INP in the simulation I force a step voltage from 0V to +15V (see the image below).

A zoomed in shot of the output voltages

As the Gain is at 0.125 the expected differential output Voltage should be 1.875V, however when I check in the simulation the result is 1.853V (see image below).

This result is the last entry in the table below.

I went through the same process with the rest of the input voltages in the table below and obtained the simulated differential output Voltage versus the expected differential output Voltage.

The Voltage delta between expected and simulated is plotted below the table (I also added the OUTP-GND and OUTN_GND deltas on the plot).

There is also a plot of the % error column.

I am trying to understand why there is a large source of error between the expected and simulated values especially from 10V upwards.

Based on the datasheet specifications I cannot come up with the same output error.

Hey Joseph, 

I recommend increasing the Vocm voltage to at least 2V. According to the datasheet, with VS = ±18V, LVDD = 5V, and LVSS = 0V. 

The requirements for VOCM = 2V to 3V. 

Can you help me understand why:

Joseph Connolly said:

I have to keep the VOCM at 1.5V in my design.

If this voltage is not available in your design, I recommend leaving the VOCM pin floating as it will provide an internal reference at mid-supply. 

Please let me know if this still has a large error %, I will take into lab and test it on the EVM. 

All the best,
Carolina 

Thank you very much, reworking the reference to 2V will require extra design work on my end that I would rather avoid but I may have to consider it.

Yes, I tried with the VS = ±18V, LVDD = 5V, and LVSS = 0V and VOCM at 2V, but I see no change to the differential output voltage.

Many thanks for your reply, I appreciate the assistance

Hey Joseph, 

Understood, I will take into lab and have some results to review to share by 11/20 end of day. 

In the meantime, could you share what % accuracy are the capacitors and resistors in your design? 

All the best,
Carolina 

Thank you very much, all capacitors are 5% and resistors are 1% tolerances

Hey Joseph, 

A small update: When I measured this in lab my results match almost perfectly your EXP column. Which leads me to think something odd is happening in the simulation. Could you please attach it? 

If not, no problem - I will rebuild the sim tomorrow.

Additionally, have you tried building this or is the data comparison just theoretical versus sim?
If you have, could I see your schematic and layout?

All the best,
Carolina

Hi Caro,

That's great, many thanks for checking this on the EVM.

Maybe there is a minor issue with the model or the simulation on my end, I have attached the simulations I used and sent them via email as I cannot seem to attach them here.

No I haven't tried to build it, its theoretical versus sim, I want to use this part in my design so wanted to see some simulations first. 

Thanks again for the great support. 

Can you confirm you used the same settings as those in my simulation i.e. 

VS+ = +18V, VS- = -18V, LVDD = 5V, LVSS = GND, A0=A1=A2=GND (Gain of 0.125), INN = GND, VOCM = 2V, INP = Step input applied.

Thank you

Hey Joseph, 

It was the conditions you outlined, except VOCM was 2.5. I can test with VOCM of 1.5V and 2V, I expect 2V to be the same, the 1.5V might be a variable. 
I can test this by EOD Monday, if needed. 

All the best,
Carolina

If you could just retest it with VOCM at 2V that is enough I have reworked my design to have VOCM at this value.

Would you be able to give me an idea of the actual % error even for one of the settings in the table it would be very helpful.

Many thanks again for the support.

Hey Joseph, 

Sorry for delay here, will post updates tomorrow. 

All the best,
Carolina

Hi Caro, 

Not at all, many thanks for getting back to me, that's great.

Thanks for doing these measurements (and the support) for me, the results look great (measured) and more in line with the datasheet. 

No problem, I have used this device in my design and look forward to using it again in the future.

Enjoy the holidays,

All the best

Joseph