THS4552 Datasheet, PDF (49/71 Pages) Texas Instruments – Dual-Channel, Low-Noise, Precision, 150-MHz, Fully Differential Amplifier

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THS4552
SBOS831 – DECEMBER 2016
One approach to increasing the phase margin when there is a feedback capacitor is to include a differential input
capacitor. This approach increases the noise gain at higher frequencies, thus creating a lower-frequency loop
gain equal to a 0-dB crossover with more phase margin. Figure 88 shows a differential input capacitor equal to
the feedback capacitor in the test circuit. This approach increases the noise gain from 1 V/V at higher
frequencies (with only a feedback capacitor) to a noise gain of 3 V/V at higher frequencies.
High-Gain, Single-Ended
to Differential Output Stage
with Feedback Pole
CF1
250 pF
RF1
2k
THS4552 Wideband,
Fully Differential Amplifier
VS+
+
5V
±
VS-
0V ⁺
±
RG1
200
VS+
±
10
VOCM
+
FDA
VIN
±
250 pF
10 nF
+
PD
10 nF
SAR ADC
Input
VS- ^VS+
10
RG2
200
RF2
2k
CF2
250 pF
Copyright © 2016, Texas Instruments Incorporated
Figure 88. Single-Ended to Differential Stage with a Feedback Pole and Differential Input Capacitor
Re-running the wideband response (using the SBOC474 TINA-TI™ simulation file) simulation illustrates in
Figure 89 that the resonance is greatly reduced with the higher noise gain at the loop gain equal to a 0-dB
crossover at a lower frequency. Although this example is only modestly peaking, good design practice is to
include a place for a differential input capacitor (even if not used) for any design using a feedback capacitor
across the feedback resistors. This recommendation applies to this simple example and to multiple feedback
active filter designs.
20
200
Gain (dB)
10
Phase (deg) 160
0
120
-10
80
-20
40
-30
0
-40
-40
-50
-80
-60
100k
1M
10M
Frequency (Hz)
100M
-120
D071
Figure 89. Gain and Phase Plot with a Differential Input Capacitor
Copyright © 2016, Texas Instruments Incorporated
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