THS4521 Datasheet, PDF (31/65 Pages) Texas Instruments – VERY LOW POWER, NEGATIVE RAIL INPUT, RAIL-TO-RAIL OUTPUT, FULLY DIFFERENTIAL AMPLIFIER

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THS4521, THS4522, THS4524
SBOS458H – DECEMBER 2008 – REVISED JUNE 2015
8.3.10 Driving Capacitive Loads
The THS4521, THS4522, and THS4524 family is designed for a nominal capacitive load of 1 pF on each output
to ground. When driving capacitive loads greater than 1 pF, it is recommended to use small resistors (R_O) in
series with the output, placed as close to the device as possible. Without R_O, capacitance on the output interacts
with the output impedance of the amplifier and causes phase shift in the loop gain of the amplifier that reduces
the phase margin. This reduction in phase margin results in frequency response peaking; overshoot, undershoot,
and/or ringing when a step or square-wave signal is applied; and may lead to instability or oscillation. Inserting
R_O isolates the phase shift from the loop gain path and restores the phase margin, but it also limits bandwidth.
Figure 64 shows the recommended values of R_O versus capacitive loads (C_L), and Figure 65 shows an
illustration of the frequency response with various values.
RECOMMENDED R_O vs C_LOAD
For Flat Frequency Response
1k
100
10 ^{VS+ = 5.0 V}
Gain = 1 V/V
R_F = 1 kW
R_L = 1 kW Differential
V_OUT = 100 mV_PP
1
10
100
1000
C_LOAD (pF)
Figure 64. Recommended Series Output Resistor Versus
Capacitive Load for Flat Frequency Response, With R_LOAD
= 1 kΩ
FREQUENCY RESPONSE vs C_LOAD
5
R_O = 150 W
0
R_O = 7.15 W
C_L = 4.7 pF
each output
-5
C_L = 1000 pF each output
-10
R_O = 37.5 W
C_L = 100 pF each output
-15
V_S+ = 5.0 V, Gain = 1 V/V
-20
R_F = 1 kW differential
R_L = 1 kW
R_O = 124 W
C_L = 10 pF
-25 ^{VOUT = 100 mVPP}
each output
0.1
1
10
100
1000
Frequency (MHz)
Figure 65. Frequency Response for Various R_O and C_L
Values, With R_LOAD = 1 kΩ
8.3.11 Audio Performance
The THS4521, THS4522, and THS4524 family provide excellent audio performance with very low quiescent
power. To show performance in the audio band, the device was tested with a SYS-2722 audio analyzer from
Audio Precision. THD+N and FFT tests were performed at 1-V_RMS output voltage. Performance is the same on
both 3.3-V and 5-V supplies. Figure 66 shows the test circuit used; see Figure 67 and Figure 68 for the
performance of the analyzer using internal loopback mode (generator) together with the THS4521.
1 kW
1 kW
V_S+
V_IN+
From
AP
Analyzer
V_{IN- Open}
PD
0.22 mF
THS452x
V_OCM
V_S-
24.9 W
24.9 W
V_OUT-
V_OUT+ ^{To AP}
Analyzer
Open
0.22 mF
1 kW
1 kW
Figure 66. THS4521 AP Analyzer Test Circuit
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Product Folder Links: THS4521 THS4522 THS4524