THS4531A_15 Datasheet, PDF (38/59 Pages) Texas Instruments – THS4531A Ultra Low-Power, Rail-to-Rail Output, Fully Differential Amplifier

THS4531A
SLOS823B – DECEMBER 2012 – REVISED JUNE 2015
9.2.3 SAR ADC Performance: THS4531A and ADS7945 Combined Performance
V_IN±
Differential
Signal from
Generator and
10 kHz BPF
2 k
100 pF
0.22 F
2 k
40.2
5V
V_OUT±
+
VOCM
±
THS4531A 1 nF
V_OUT+
ADS7945
2 k
2 k
40.2
V_IN+
100 pF
To Data
Analysis
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Figure 90. THS4531A and ADS7945 Test Circuit
9.2.3.1 Design Requirements
To show achievable performance with a high performance SAR ADC, the THS4531A is tested as the drive
amplifier for the ADS7945. The ADS7945 is a 14-bit, SAR ADC that offers excellent AC and DC performance,
with low power and small size. The circuit shown in Figure 90 is used to test the performance. Data was taken
using the ADS7945 at 2MSPS with input frequency of 10 kHz and signal level 0.5 dB below full scale. The FFT
plot of the spectral performance is in Figure 91. A summary of the FFT analysis results are in Table 8 along with
ADS7945 typical data sheet performance at f_S = 2 MSPS. Refer to the data sheet for more information.
9.2.3.2 Detailed Design Procedure
The standard ADS7945 EVM and THS4531A EVM are modified to implement the schematic in Figure 90 and
used to test the performance of the THS4531A as a drive amplifier. With single supply 5 V supply the output
common-mode of the THS4531A defaults to +2.5 V as required at the input of the ADS7945 so the V_OCM input of
the THS4531A simply bypassed to GND with 0.22-µF capacitor. The summary of results of the FFT analysis
versus typical data sheet performance shown in Table 8 show that the THS4531A will make an excellent drive
amplifier for this ADC.
9.2.3.3 Application Curve
0
−10
−20
−30
−40
−50
−60
−70
−80
−90
−100
−110
−120
−130
−140
1k
V_S = 5 V
G = 1 V/V
R_F = 2 kΩ
V_OUT = −0.5 dBFS
10k
100k
Frequency (Hz)
1M
G077
Figure 91. THS4531A and ADS7945 Test Circuit
CONFIGURATION
THS4531A + ADS7945
ADS7945 Data Sheet (typ)
Table 8. 10-kHz FFT Analysis Summary
TONE
10 kHz
10 kHz
SIGNAL
–0.5 dBFS
–0.5 dBFS
SNR
83 dBc
84 dBc
THD
–93 dBc
–92 dBc
SFDR
96 dBc
94 dBc
38
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