TINA/Spice/PGA870: PGA usage handholding

eshwar j

Part Number: PGA870
Other Parts Discussed in Thread: TINA-TI,

Tool/software: TINA-TI or Spice Models

Hi, i have simulated PGA870 with ±2.5 split supply and rest of the connected capacitors are used by referring datasheet.

I want to excite the circuit with 2mV, 2Meg and simulate the PGA for various gains configurations

I have tried everything in accordance with my understanding from the data sheet. I am clueless about extracting any control information from gain simulation in TINA

Please suggest any methods if i am doing something wrongly

over 6 years ago

0 Jacob Freet over 6 years ago

TI__Expert 8000 points

Hi,

I'm going to move this thread to our spice model support forum.

Regards,

0 Micah B Brouwer over 6 years ago

TI__Intellectual 1910 points

Hello,

I recommend taking a look at the TINA reference design circuit that is available on the PGA870 product page under "tools and software." I have included it in this post. I tried switching the supplies from +5V to +/- 2.5V and did not achieve the same performance. Perhaps use +5V supply while modeling takes a look at the circuit?

slom168.TSC

0 eshwar j over 6 years ago in reply to Micah B Brouwer

Prodigy 110 points

Hi,

The attachment provided was of great help

However i saw that the output waveform is square wave type even when the input waveform is a sine. The output waveform must be sine

Please advise

0 Micah B Brouwer over 6 years ago in reply to eshwar j

TI__Intellectual 1910 points

Hello,

If you take a look at OUTP across C1 you'll see that the single-ended output is 1.1V to 3.9V. I think you may have removed OUTP in your schematic, it is hard to tell.

It looks like you are in a gain of 20 dB, which is a gain of 10 V/V. If you input a 1Vpp sine wave, the device will try to output a 10Vpp sine wave. However, the output limitations (pg 4 of the datasheet) state that the typical maximum output voltage high is 3.7V. In the case of the simulation, the output reaches 3.9V, which is the the highest it can go. The output is being railed, and that is why it looks like a square wave.

Try again with a lower gain setting or decrease the amplitude of the input signal. If you change the input sine wave to 100mVpp you will see a sine wave on the output.

0 eshwar j over 6 years ago in reply to Micah B Brouwer

Prodigy 110 points

This thread was really helpful, Thanks

0 eshwar j over 6 years ago in reply to Micah B Brouwer

Prodigy 110 points

Hi, in the attachment provided by you, i saw that the PGA has been operated on single 5v Vcc and Vee at 0v. Yet the output is bipolar. Can you please explain this fact so that we can proceed with this PGA for our design

0 Micah B Brouwer over 6 years ago in reply to eshwar j

TI__Intellectual 1910 points

Hello,

That is because the output is measured differentially across RLdiff. If each output pin was measured with respect to ground, they would both be single-ended, as shown in OUTN and OUTP below. The input in the below simulation is a 200mVpp 50 MHz square wave and the inverting and non-inverting inputs are shown as INN and INP. The common-mode voltage of the inputs is mid-supply, or 2.5 V in this case.

1351.slom168.TSC

For more information on fully differential amplifiers, I recommend checking out TI Precision Labs video series on FDAs available here.

0 eshwar j over 6 years ago in reply to Micah B Brouwer

Prodigy 110 points

Hi,
Can the output be consolidate from the OUTN and OUTP by any means to obtain a coherent output as like in a normal opamp from a single OUT with respect to ground?

Thanks

0 Micah B Brouwer over 6 years ago in reply to eshwar j

TI__Intellectual 1910 points

Hello,

If you want a single-ended output rather than a differential output, I suggest either using a balun or using a different amplifier than has a single-ended output

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TINA/Spice/PGA870: PGA usage handholding