THS4524-EP Datasheet, PDF (30/48 Pages) Texas Instruments – VERY LOW POWER, NEGATIVE RAIL INPUT, RAIL-TO-RAIL OUTPUT, FULLY DIFFERENTIAL AMPLIFIER

THS4524-EP
SBOS609A – JUNE 2012 – REVISED AUGUST 2013
Driving Capacitive Loads
The THS4524 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 67 shows the
recommended values of R_O versus capacitive loads
(C_L), and Figure 68 shows an illustration of the
frequency response with various values.
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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 68. Frequency Response for Various R_O
and C_L Values, with R_LOAD = 1 kΩ
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 67. Recommended Series Output Resistor
versus Capacitive Load for Flat Frequency
Response, with R_LOAD = 1 kΩ
Audio Performance
The THS4524 provides 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 69 shows the test circuit used;
see Figure 70 and Figure 71 for the performance of
the analyzer using internal loopback mode
(generator) together with the THS4524.
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 69. THS4524 AP Analyzer Test Circuit
30
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