THS4552 Datasheet, PDF (28/71 Pages) Texas Instruments – Dual-Channel, Low-Noise, Precision, 150-MHz, Fully Differential Amplifier

THS4552
SBOS831 – DECEMBER 2016
www.ti.com
9.3 Feature Description
9.3.1 Differential Open-Loop Gain and Output Impedance
The most important elements to the closed-loop performance are the open-loop gain and open-loop output
impedance. Figure 66 and Figure 67 show the simulated differential open-loop gain and phase from the
differential inputs to the differential outputs with no load and with a 100-Ω load. Operating with no load removes
any effect introduced by the open-loop output impedance to a finite load. This A_OL simulation removes the 0.6-pF
internal feedback capacitors to isolate the forward path gain and phase (see Figure 102). The 0.6-pF capacitance
becomes part of the feedback network that sets the noise gain and phase combined with the external elements.
The simulated differential open-loop output impedance is shown in Figure 68.
80
100 : Load
70
No Load
60
50
40
30
20
10
0
-10
-20
100k
1M
10M
100M
1G
Frequency (Hz)
D063
Figure 66. No-Load and 100-Ω Loaded A_OL Gain
10000
-90
-100
-110
-120
-130
-140
-150
-160
-170
-180
-190
-200
-210
100k
No Load
100 : Load
1M
10M
100M
Frequency (Hz)
Figure 67. No-Load and 100-Ω A_OL Phase
1G
D064
1000
100
10
1
10 100 1k 10k 100k 1M 10M 100M 1G
Frequency (Hz)
D062
Figure 68. Differential Open-Loop Output Impedance
This impedance combines with the load to shift the apparent open-loop gain and phase to the output pins when
the load changes. The rail-to-rail output stage shows a very high impedance at low frequencies that reduces with
frequency to a lower midrange value and then peaks again at higher frequencies. The maximum value at low
frequencies is set by the common-mode sensing resistors to be a 10.5-kΩ dc value (see the Functional Block
Diagram section). This high impedance at a low frequency is significantly reduced in closed-loop operation by the
loop gain, as shown in the closed-loop output impedance of Figure 38. Figure 66 compares the no load A_OL gain
to the A_OL gain driving a 100-Ω load that shows the effect of the output impedance. The heavier loads pull the
A_OL gain down faster to lower crossovers with more phase shift at the lower frequencies.
The much faster phase rolloff for the 100-Ω differential load explains the greater peaked response illustrated in
Figure 4 and Figure 22 when the load decreases. This same effect happens for the RC loads common with
converter interface designs. Use the TINA-TI™ model to verify loop phase margin in any design.
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