Signal conditioning
RC filter is used, the roll off should occur at 5kHz. For RC values, 10nF and 3.16kΩ were chosen since they meet the criteria above and they are also standard values. As mentioned above, the –3dB of the low-
pass filter is 5kHz. Since the buffer is supplying negative feedback for the op amp inside the AD8479, as the low-pass filter begins to roll off, the gain to the output of the AD8479 will increase for f > 5kHz. Since the AD8479 output increases at 20dB/decade once the low-pass filter star ts rolling off, the resulting output of the filter and therefore the output of the buffer will be flat. Taking the system output at the buffer output will provide overall bandwidth limited only by the bandwidth and output range of the AD8479. This limitation is due to the increased gain to the AD8479 output for frequencies greater than 5kHz, therefore this circuit has an input voltage range vs. frequency trade-off for frequencies at and above 5kHz. For example, a 30Vp-p input at 150kHz will have a –6dB gain to the AD8479 output resulting in 15Vp-p, which is approaching the full-power bandwidth of the AD8479. The oscilloscope capture in Figure 4 shows
Figure 6. AD8479: Gain of 1/60 pulse response.
the result of the funnel amplifier configuration of the AD8479. The input signal is 100Hz, 1200Vp-p, which is displayed as Channel 1 and is attenuated by a factor of 100 to avoid damaging the oscilloscope. Channel 2 is the output of the buffer amplifier and the results are exactly as desired. For a 1200Vp-p input, the funnel amplifier displays 20Vp-p. The oscilloscope capture in Figure 5 shows
the results of the 30Vp-p, 100kHz input signal. Just as in Figure 4, the funnel circuit provides the same attenuation of 1/60 at 100kHz. Figure 6 displays the step response of
the AD8479 funnel circuit. Driving the inputs with a 15Vp-p square wave results in a 250mVp-p step response that is settled within a few microseconds. Since the AD8479 funnel amplifier
configuration doesn’t gain up the differential signal as the standard AD8479 does, the noise is reduced. For the funnel amplifier configuration, the spectral noise density at 100Hz is 27nV/√Hz with 580nV peak-to-peak voltage noise from 0.1Hz to 10Hz. As you can see, these noise values are approximately 1/60 of what is listed in the AD8479 data sheet, therefore the filter and buffer have a negligible effect on the noise. This is due to the fact that in a two-stage amplifier circuit the noise and offset of the second stage is divided down by the gain of the first stage. Since the gain from the AD8479 Ref– pin to the AD8479 output is –59, this (minus 1) is the factor by which the buffer noise and offset will be reduced. Two of the key specifications of the
Figure 7. AD8479: Gain of 1/60 peak-to-peak noise (nV) 0.1 Hz to 10 Hz.
AD8479 are the offset voltage and the common-mode rejection ratio. Since the noise gain of the AD8479 is now approximately 1 at dc, the offset from the op amp inside the AD8479 will be 1/60th of the offset specified in the AD8479 data sheet, which is ±1mV for
54 August 2022 Instrumentation Monthly
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