Hello everyone,
I would like to use the AD8411A to amplify the drain-source voltage of the high-side transistor in a buck-converter to measure the current. The converter operates at 1.5 MHz, so a fast amplifier is needed. The input is 48 V nominal so the AD8411A would be perfectly suited for this task, if its gain was lower. I need around 10 V/V but there is no need for it to be very accurate. It's more about detecting if something really goes wrong and to get some insights during prototyping.
So I was thinking about putting additional resistors in series with the input as shown below. According to the schematic in the datasheet, the internal divider is 12k/600k, so adding 48k should reduce the gain to 10 V/V. Of course the internal resistance is going to be very inaccurate so the gain might end up being 8 or 12 V/V but this is no problem as it can be calibrated. Also, the input offset current of will cause some gain-error but the variation of Rds,on is likely much bigger. The Zener diode is added to prevent too large differential input voltage.
Assuming the GBW of the internal amplifier is still the limiting factor and not the filter of the external resistor and input capacitance, the BW should increase by 5, right? Do you see any issues with this design? In LTspice, the gain is not significantly reduced when adding the series resistors so either there is something I didn't consider or the model isn't good enough.
It's also interesting to look at this patent from Analog which I assume is implemented in the AD8411A and explains the large input bias current: US10312865B2 - Voltage range extension for difference amplifier - Google Patents
Best regards
Adrian
