THS4521_10 Datasheet, PDF (28/48 Pages) Texas Instruments – VERY LOW POWER, NEGATIVE RAIL INPUT, RAIL-TO-RAIL OUTPUT, FULLY DIFFERENTIAL AMPLIFIER

THS4521
THS4522
THS4524
SBOS458D – DECEMBER 2008 – REVISED AUGUST 2010
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 66 shows the recommended
values of R_O versus capacitive loads (C_L), and
Figure 67 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
C_LOAD (pF)
1000
Figure 66. Recommended Series Output Resistor
versus Capacitive Load for Flat Frequency
Response, with R_LOAD = 1 kΩ
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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
Frequency (MHz)
1000
Figure 67. Frequency Response for Various R_O
and C_L Values, with R_LOAD = 1 kΩ
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 68 shows the test circuit used; see
Figure 69 and Figure 70 for the performance of the
analyzer using internal loopback mode (generator)
together with the THS4521.
V_IN+
From
AP
Analyzer
V_{IN- Open}
1 kW
PD
0.22 mF
1 kW
1 kW
V_S+
THS452x
V_OCM
V_S-
1 kW
24.9 W
24.9 W
V_OUT-
V_OUT+ ^{To AP}
Analyzer
Open
0.22 mF
Figure 68. THS4521 AP Analyzer Test Circuit
28
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