THS4521 Datasheet, PDF (32/65 Pages) Texas Instruments – VERY LOW POWER, NEGATIVE RAIL INPUT, RAIL-TO-RAIL OUTPUT, FULLY DIFFERENTIAL AMPLIFIER

THS4521, THS4522, THS4524
SBOS458H – DECEMBER 2008 – REVISED JUNE 2015
www.ti.com
Note that the harmonic distortion performance is very close to the same with and without the device meaning the
THS4521 performance is actually much better than can be directly measured by this method. The actual device
performance can be estimated by placing the device in a large noise gain and using the reduction in loop gain
correction. The THS4521 is placed in a noise gain of 101 by adding a 10-Ω resistor directly across the input
terminals of the circuit shown in Figure 66. This test was performed using the AP instrument as both the signal
source and the analyzer. The second-order harmonic distortion at 1 kHz is estimated to be –122 dBc with V_O =
1V_RMS; third-order harmonic distortion is estimated to be –141 dBc. The third-order harmonic distortion result
matches exactly with design simulations, but the second-order harmonic distortion is about 10 dB worse. This
result is not unexpected because second-order harmonic distortion performance with a differential signal
depends heavily on cancellation as a result of the differential nature of the signal, which depends on board
layout, bypass capacitors, external cabling, and so forth. Note that the circuit of Figure 66 is also used to
measure crosstalk between channels.
The THS4521 shows even better THD+N performance when driving higher amplitude output, such as 5 V_PP that
is more typical when driving an ADC. To show performance with an extended frequency range, higher gain, and
higher amplitude, the device was tested with 5 V_PP up to 80 kHz with the AP. Figure 69 shows the resulting
THD+N graph with no weighting.
TOTAL HARMONIC DISTORTION + NOISE
THS4521 Measured on AP Analyzer
-50
-60
-70
-80
-90
-100
-110
THS4521
Signal Generator
-120
0
5
10
15
20
Frequency (kHz)
Figure 67. THS4521 1-V_RMS 20-Hz to 20-kHz Thd+N
10-kHz OUTPUT SPECTRUM
THS4521 on AP Analyzer
10
0
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
-110
-120
-130
-140
V_S+ = 5.0 V
G = 1 V/V
R_F = 1 kW
V_OUT = 1 V_RMS
Generator
THS4521
0
5 k 10 k 15 k 20 k 25 k 30 k 35 k
Frequency (Hz)
Figure 68. THS4521 1-V_RMS 10-kHz FFT Plot
TOTAL HARMONIC DISTORTION + NOISE
vs FREQUENCY (No Weighting)
-95
-97
-99
-101
-103
-105
-107
-109
-111
-113
-115
10
100
1k
10 k
Frequency (Hz)
100 k
Figure 69. Thd+N (No Weighting) on Ap, 80-kHz Bandwidth at G = 1 With 5-V_PP Output
32
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