THS4552 Datasheet, PDF (47/71 Pages) Texas Instruments – Dual-Channel, Low-Noise, Precision, 150-MHz, Fully Differential Amplifier

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THS4552
SBOS831 – DECEMBER 2016
The 16.8 pF across the inputs is really a total of 36 pF for a single-ended design from Equation 13 reduced by
half and then the 1.2-pF internal capacitance is removed.
These two designs (with and without the compensation capacitors) were both bench tested and simulated using
the THS4552 TINA-TI™ model, which resulted in Figure 85. The TINA-TI™ simulation files used for Figure 85
are available both without the compensation capacitors and with the capacitors in place.
-12
-14
-16
-18
-20
-22
-24
-26
-28
-30
1M
Bench, wo Capacitors
Bench, with Capacitors
TINA-TI•, wo Capacitors
TINA-TI•, with Capacitors
10M
Frequency (Hz)
100M
D067
Figure 85. Attenuator Response Shapes With and Without External Capacitors
This approach does a good job of flattening the response for what starts out as a low phase margin attenuator
application. The simulation model does a very good job of predicting the peaking and showing the same
improvement with the external capacitors (both give a flat, approximately 24-MHz, closed-loop bandwidth for the
gain of 0.1-V/V design). The output noise starts to peak up (because of the noise gain shaping of the capacitors)
above 3.5 MHz in this example. These stages normally drive the RC filter at the input of a SAR ADC that filters
off the noise peaking above 3.5 MHz.
10.1.9 The Effect of Adding a Feedback Capacitor
Adding a feedback capacitor to band-limit the signal path is very common in lower frequency designs. This
approach is very effective for the signal path gain but does create the potential for high-frequency peaking and
oscillation for a wideband device such as the THS4552. The feedback capacitor by itself takes the noise gain to
1 V/V at high frequencies. Depending on the frequency where the noise gain goes to 1V/V, and what added
phase margin reduction may already be in place resulting from the load RC, the feedback capacitors can cause
instability.
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