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I am having issues simulating the THS7001 programmable gain amplifier (PGA). I have the following setup:

enter image description here

I added a voltage divider at the input R11/(R10+R11) = 0.5, and configured the pre-amp stage of the THS7001 as a voltage follower.

When I apply a 10 kHz, 1 V signal at the input of the voltage divider, I measure a voltage of ~360 mV at Vin.

Voltage at Vin with R11 = R10 = 100kΩ

Oddly enough, when I reduce the resistances R10 and R11 to 1 kΩ the problem seems to go away, and I measure the expected ~500 mV at Vin:

Voltage at Vin with R11 = R10 = 1kΩ

The same happens when I increase R10 and R11 to high resistances (1 MΩ):

Voltage at Vin with R11 = R10 = 1MΩ

It seems that the issue is only with the pre-amp stage of the THS7001.

I imported the PSpice model of the THS7001 (Rev B) from Texas Instruments to my LTspice simulation using instructions found here.

Any advice on why I am getting this weird behavior will be appreciated.

ocrdu
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Adriaan Sadie
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    Are you sure you're using 1 megaohm? `1M` means 1 milli in SPICE. You need to type in `1meg` to get 1 mega. – Ste Kulov Feb 21 '23 at 17:54
  • I have indeed used 1M in the simulation. Changing to 1meg now causes a much larger voltage error, which makes sense. – Adriaan Sadie Feb 22 '23 at 05:42

1 Answers1

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From the datasheet: enter image description here

The output impedance from your divider is 50K, so these currents cause a significant voltage error. When you drop to 1K resistors, the the output impedance is 500 ohms, so the currents cause relatively little problem. As @Ste Kulov points out, 1 milliohm resistors make the problem almost completely disappeaar.

These amplifiers are designed for high frequency, relatively low impedance signals.

John Birckhead
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  • Thanks. This makes a lot of sense. When calculating the voltage drop based on the bias current in the data sheet, it corresponds to the signal I am measuring in the simulation. I guess I will drop the value of the voltage divider resistors for now. – Adriaan Sadie Feb 22 '23 at 05:46