THS4541 datasheet(24/64 Pages) TI1

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Frequency (Hz)

10M

100M

D062

Aol Gain

Gain = 0.1

Gain = 1

Gain = 2

‡

Rf Cf

Cin

Rf Cf

402

FDA

Output

200 fF

221

Vout

Cin

1.7 pF

Vin

THS4541-Q1

SLOS930B – NOVEMBER 2015 – REVISED NOVEMBER 2019

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Product Folder Links: THS4541-Q1

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NG is critically important for bandwidth and all output error terms (such as DC offset and noise). For lower-speed

devices, normally only the DC noise gain is considered. However, for the THS4541-Q1, with loop gain crossover

at greater than 300 MHz, the feedback network produces a parasitic pole to the differential summing junctions

that causes the noise gain to increase with frequency. This pole causes a lower crossover frequency than might

be expected with added phase shift around the loop. Consider the feedback network (single-ended) of Figure 65,

showing a parasitic 0.2 pF on the feedback 402-Ω resistor. The 0.85-pF differential input capacitance of the

THS4541-Q1 is converted to single-ended as a 1.7-pF parasitic for this single-sided analysis circuit (the Rg

shown is Rg2 in Figure 61).

Figure 65. Feedback Network for the Gain of 2 Configuration Using 402 Ω and Matching to a 50-Ω Source

The response shape from Vout to Vin in Figure 65 has a pole and then a zero. To describe NG, invert the

Laplace transform of Vin and Vout from Figure 65 to provide the frequency-dependent NG response of

Equation 1, where a zero comes in first and then a pole.

(1)

The zero location is key. Using the gain of 2 values of Figure 65, the estimated zero in the NG is 588 MHz.

Limiting the parasitic capacitance at the summing junctions, either differentially or signal-ended, to a ground or

power plane is critical in board layouts.

Using this feedback model, and the open-loop gain and phase data for the THS4541-Q1, allows the Aol and NG

curves over frequency to be drawn, as shown in Figure 66, where the peaking in the noise gain pulls the

intersection point back in frequency.

Figure 66. Aol and Noise Gain Plots for the Lower Gains of Figure 61

THS4541 Datasheet(PDF) 24 Page - Texas Instruments

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