THS4541_16 Datasheet, PDF (24/61 Pages) Texas Instruments – THS4541 Negative Rail Input, Rail-to-Rail Output, Precision, 850-MHz Fully Differential Amplifier

THS4541
SLOS375A – AUGUST 2014 – REVISED SEPTEMBER 2014
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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, 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 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).
Cf
200 fF
Rf
402
+ ^FDA
Vout
Output
±
Pin
Rg
221
Vin
Cin
1.7 pF
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  ^Rf
NG
§¨©_{1 }
Cin
Cf
·
¸¹
s

Rf
s
Rg
Cf  Cin
1
Rf W Cf
(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, 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.
40
Aol Gain
35
Gain = 0.1
Gain = 1
30
Gain = 2
25
20
15
10
5
0
10M
100M
Frequency (Hz)
1G
D062
Figure 66. Aol and Noise Gain Plots for the Lower Gains of Figure 61
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