OPA4820ID Datasheet, PDF (16/34 Pages) Texas Instruments – Quad, Unity-Gain Stable, Low-Noise, Voltage-Feedback Operational Amplifier

OPA4820
SBOS317D − SEPTEMBER 2004 − REVISED AUGUST 2008
DC-COUPLED SINGLE-TO-DIFFERENTIAL
CONVERSION
The previous differential output circuits were set up to
receive a differential input as well as provide a differential
output. Figure 7 shows one way to provide a single to
differential conversion, with DC coupling, and independent
output common-mode control using a quad op amp.
The circuit of Figure 7 provides several useful features for
isolating the input signal from the final outputs. Using the
first amplifier as a simple noninverting stage gives an
independent adjustment on R_I (to set the source loading)
while the gain can be easily adjusting in this stage using
the R_G resistor. The next stage allows a separate output
common-mode level to be set up. The desired output
common-mode voltage, V_CM, is cut in half and applied to
the noninverting input of the 2nd stage. The signal path in
this stage sees a gain of −1 while this (1/2 × V_CM) voltage
sees a gain of +2. The output of this 2nd stage is then the
original common-mode voltage plus the inverted signal
from the output of the first stage. The 2nd stage output
appears directly at the output of the noninverting final
stage. The inverting node of the inverting output stage is
also biased to the common-mode voltage, equal to the
common-mode voltage appearing at the output of the 2nd
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stage, creating no current flow and placing the desired
V_CM at the output of this stage as well. Both the positive
and negative output are shown in Figure 8.
LOW-POWER, DIFFERENTIAL I/O,
4th-ORDER ACTIVE FILTER
The OPA4820 can give a very capable gain block for active
filters. The quad design lends itself very well to differential
active filters. Where the filter topology is looking for a
simple gain function to implement the filter, the
noninverting configuration is preferred to isolate the filter
elements from the gain elements in the design. Figure 9
shows an example of a 10MHz, 4th-order Butterworth,
low-pass Sallen-Key filter. The design places the higher Q
stage first to allow the lower Q 2nd stage to roll off the
peaked noise of the first stage. The resistor values have
been adjusted slightly to account for the amplifier group
delay.
While this circuit is bipolar, using ±5V supplies, it can easily
be adapted to single-supply operation. This will add two
real zeroes in the response, transforming this circuit into a
bandpass. The frequency response for the filter of Figure 9
is shown in Figure 10.
V_I
R_I
1/4
OPA4820
402Ω
R_G
0.1µF
+5V
1.5V
V_CM
200Ω
750Ω _VCM
2
750Ω
1/4
OPA4820
402Ω
1/4
OPA4820
402Ω
750Ω
750Ω
50Ω
200Ω
1/4
OPA4820
250Ω
+V_OUT
=
V_CM
+
V_I (1
+
402Ω
R_G
)
−V_OUT = V_CM −
402Ω
V_I (1 + _RG )
−5V
Figure 7. Precision, DC-Coupled, Single-to-Differential Conversion
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