Design Goals

Design Description

This design uses a fully differential amplifier (FDA) as a differential input to differential output amplifier.

Design Notes

1. The ratio R2/R1, equal to R4/R3, sets the gain of the amplifier.
2. For a given supply, the output swing for and FDA is twice that of a single ended amplifier. This is because a fully differential amplifier swings both terminals of the output, instead of swinging one and fixing the other to either ground or a Vref. The minimum voltage of an FDA is therefore achieved when Vout+ is held at the negative rail and Vout- is held at the positive rail, and the maximum is achieved when Vout+ is held at the positive rail and Vout- is held at the negative rail.
3. FDAs are useful for noise sensitive signals, since noise coupling equally into both inputs will not be amplified, as is the case in a single ended signal referenced to ground.
4. The output voltages will be centered about the output common-mode voltage set by Vocm.
5. Both feedback paths should be kept symmetrical in layout.

Design Steps

• Set the ratio R2/ R1 to select the AC voltage gain. To keep the feedback paths balanced,
  
• Given the output rails of 9.8 V and 0.2 V for Vs = 10 V, verify that 16 Vpp falls within the output range available for V_ocm = 5 V.

  In normal operation:

  
• Rearrange to solve for each output voltage in edge conditions
  
• Verifying for Vout = +8 V and V_ocm = +5 V,
  
• Verifying for Vout = -8 V and V_ocm = +5 V,
  

  Note that the maximum swing possible is:

  
• Use the input common mode voltage range of the amplifier and the feedback resistor divider to find the signal input range when the output range is 1 V to 9 V. Due to symmetry, calculation of one side is sufficient.
  

Design Simulations

AC Simulation Results

Transient Simulation Results

Design References

Texas Instruments, High-Q Active Differential Bandpass Filter Reference Design for Instrumentation Qualification, TIDA-01036 tool folder

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