THS4521_10 Datasheet, PDF (29/48 Pages) Texas Instruments – VERY LOW POWER, NEGATIVE RAIL INPUT, RAIL-TO-RAIL OUTPUT, FULLY DIFFERENTIAL AMPLIFIER

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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
meathod. 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 68. 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 68 is also used to measure
crosstalk between channels.
-50
-60
-70
-80
-90
-100
-110
-120
0
TOTAL HARMONIC DISTORTION + NOISE
THS4521 Measured on AP Analyzer
THS4521
Signal Generator
5
10
15
20
Frequency (kHz)
Figure 69. THS4521 1-V_RMS 20-Hz
to 20-kHz THD+N
THS4521
THS4522
THS4524
SBOS458D – DECEMBER 2008 – REVISED AUGUST 2010
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 70. THS4521 1-V_RMS 10-kHz FFT Plot
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 71 shows the resulting THD+N graph
with no weighting.
-95
-97
-99
-101
-103
-105
-107
-109
-111
-113
-115
10
TOTAL HARMONIC DISTORTION + NOISE
vs FREQUENCY (No Weighting)
100
1k
10 k
Frequency (Hz)
100 k
Figure 71. THD+N (No Weighting) on AP, 80-kHz
Bandwidth at G = 1 with 5-V_PP Output
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Product Folder Link(s): THS4521 THS4522 THS4524