Electronic – Attenuating a 10V Sin wave [Circuit design]

circuit-designimpedance-matchingoperational-amplifier

About a year ago I designed a circuit that takes a 10V analogue signal and converts it to a 5V level and gets read by an ADC etc.

Here's the original circuit:
The AD620 has external pins for a single gain resistor, it has a gain of 1 in the circuit

schematic

simulate this circuit – Schematic created using CircuitLab

I've decided to revisit some of my old designs and was finding ways to improve upon what I previously built.

  • With input limits of 0V and 10V my clamping diodes aren't needed at all, as the maximum limits I get after the voltage divider are 0V and 5V anyway.
  • I buffer the singal, divide it down and then buffer it again which I think could be better implemented
  • There is no impedance matching on the input of the AD620

I've redesigned the circuit and removed some components and added another resistor on the input and have come up with the following circuit

schematic

simulate this circuit

My signal source can supply 150mA so shouldn't have a problem driving the low impedance resistor.

Question – I guess my actual question is this, are these two circuits comparable? Or are there any better solutions?
Any other feedback or issues you see with either of my designs feel free to point out.

Best Answer

What you've actually made between your signal line & your AD8676 in the second diagram is a "pi attenuator."

While a pi attenuator could actually be used quite well in this instance, I think yours has the wrong resistor values.

Here's a link to a calculator made specifically for making impedance-matched pi attenuator circuits like that: Pi Attenuator Calculator

using that calculator, if your op-amp had an input impedance of 50ohms, the following would work:

schematic

simulate this circuit – Schematic created using CircuitLab
Since, however, the input of the OA is closer to 1Mohm than 50ohm, you'll probably want to do a little checking on "Impedance Matching Pi Networks" if you need to avoid reflections on the incoming signal cable.

EDIT

Or, if we stop over-complicating things on ourselves, using the fact that the op-amp's input can be considered infinite-impedance, compared to the 50-ohm required load, you could use this even simpler circuit:

schematic

simulate this circuit

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