THS4551 Datasheet, PDF (46/70 Pages) Texas Instruments – Low-Noise, Precision, 150-MHz, Fully Differential Amplifier

THS4551
SBOS778A – APRIL 2016 – REVISED AUGUST 2016
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The response shape must be probed at the FDA output pins before the added RC pole to the SAR input.
Running a wideband sweep with the THS4551 TINA-TI™ model shows a resonance at 50 MHz in Figure 84
resulting from the feedback capacitor.
20
200
Gain (dB)
15
Phase (deg) ¹⁷⁵
10
150
5
125
0
100
-5
75
-10
50
-15
25
-20
0
-25
-25
-30
100k
1M
10M
Frequency (Hz)
100M
-50
D070
Figure 84. Gain and Phase Plot with a Feedback Pole
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 85 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
THS4551 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 85. Single-Ended to Differential Stage with a Feedback Pole and Differential Input Capacitor
Re-running the wideband response simulation illustrates in Figure 86 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.
46
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