THS1401_15 Datasheet, PDF (18/28 Pages) Texas Instruments – 14-BIT, 1/3/8 MSPS, DSP-COMPATIBLE ANALOG-TO-DIGITAL

THS1401
THS1403
THS1408
SLAS248D − DECEMBER 1999 − REVISED SEPTEMBER 2005
10 kΩ 10 kΩ
−
+
22 Ω
10 kΩ + 10 kΩ
100 pF
100 pF
10 kΩ
IN+ ^REF+
THS1401/3/8
IN− _REF−
10 kΩ
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Figure 21. Single-Ended With Level Shift
The following table shows the input voltages for negative full-scale output, zero output, and positive full-scale
output:
∆IN+ [V]
OUTPUT
−∆REF
− full scale
0
0
∆REF
+ full scale
Note that the resistors of the op-amp and the op-amp all introduce gain and offset errors. Those errors can be
trimmed by varying the values of the resistors.
Because of the added offset, the op-amp does not necessarily operate in the best region of its transfer curve
(best linearity around zero) and therefore may introduce unacceptable distortion. For ac signals, an alternative
is described in the following section.
APPLICATION INFORMATION
AC-coupled single-ended configuration
If the application does not require the signal bandwidth to include dc, the level shift shown in Figure 21 is not
necessary.
10 kΩ
10 kΩ
10 kΩ 10 kΩ
−
10 nF
+
22 Ω
100 pF
IN+ ^REF+
THS1401/3/8
100 pF
IN− _REF−
10 kΩ
10 kΩ
Figure 22. Single-Ended With Level Shift
Because the signal swing on the op-amp is centered around ground, it is more likely that the signal stays within
the linear region of the op-amp transfer function, thus increasing the overall ac performance.
IN [VPEAK]
−∆REF
0
∆REF
OUTPUT [PEAK]
− full scale
0
+ full scale
Compared to the transformer-coupled configuration, the swing on IN− is twice as big, which can decrease the
ac performance (SNR, SFD, and THD).
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