THS4521 datasheet(25/66 Pages) TI2

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To assess closed-loop bandwidth and peaking, the noise-gain phase must be subtracted from the THS4541 Aol

phase to obtain the total phase around the loop, as shown in Figure 7-7.

Frequency (Hz)

-180

-170

-160

-150

-140

-130

-120

-110

-100

-90

10M

100M

D063

Aol Phase

Gain = 0.1

Gain = 1

Gain = 2

Figure 7-7. Loop-Gain Phase for the Three Lower Gains of Figure 6-1

From Figure 7-6 and Figure 7-7, using Table 7-2, tabulate the loop-gain crossover frequency and phase margin

at these crossovers to explain the response shapes of Figure 6-1.

Table 7-2. Estimated Crossover Frequency and Phase Margin for Gains of 0.1, 1, and 2 in Figure 6-1

GAIN

DC NG (V/V)

0-dB LG (MHz)

PHASE MARGIN (°)

0.1

1.1

457

1.94

380

2.85

302

From these crossover (or 0-dB loop gain) frequencies, a good approximation for the resulting f–3dB is to multiply

the crossover frequencies by 1.6 when the phase margin is less than 65°. Ideally, a 65° phase margin at

loop-gain crossover provides a flat Butterworth closed-loop response. The 59° phase margin for the gain of 2

setting explains the nearly flat response for this condition with 1.6 × 302 MHz = 483 MHz, estimated with f–3dB

closely matching the measured 500-MHz SSBW.

The very low phase margin in the attenuator setting at 0.1 V/V explains the highly peaked response in Figure

6-1. This peaking can be easily compensated, as shown in Section 9.2.1, using feedback capacitors and a

differential capacitor across the inputs.

Considering the noise gain zero as part of the loop-gain analysis shows the importance of using relatively-low,

feedback-resistor values and minimizing layout parasitic capacitance on the input pins of the THS4541 to reduce

the effects of this feedback pole. The TINA model does a good job of predicting these issues (the model includes

the 0.85 pF differential internal capacitance); add any estimated external parasitic capacitance on the summing

junctions in simulation to predict the response shape more accurately.

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THS4541

SLOS375B – AUGUST 2014 – REVISED FEBRUARY 2024

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