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Using Differential Amplifier Drive Circuits
R
G
R
F
R
F
R
FIL
R
FIL
C
FIL
C
FIL
R
G
0.1 Fm
0.1 Fm
0.1 Fm
0.1 Fm
0.1 Fm
0.1 Fm
0.1 Fm
10 Fm
10 Fm
R
S
R
S T
||R
R
T
+V
S
CM
INP
INM
ADS6xxx
THS4509
VCM
500 W
200 W
200 W
5 W
5 W
500 W
0.1 Fm
–V
S
S0259-01
INPUT COMMON MODE
155 mAxFs
125 MSPS
(1)
ADS6424
ADS6423
ADS6422
SLAS532A – MAY 2007 – REVISED JUNE 2007
APPLICATION INFORMATION (continued)
Figure 69 shows a drive ciruit using a differential amplifier (TI's THS4509) to convert a single-ended input to
differential output that can be interfaced to the ADC input pins. In addition to the single-ended to differential
conversion, the amplifier also provides gain (10dB in Figure 69 ). As shown in the figure, R
FIL
helps to isolate the
amplifier output from the switching inputs of the ADC. Together with C
FIL
, it also forms a low-pass filter that
bandlimits the noise (and signal) at the ADC input. As the amplifier outputs are ac-coupled, the common-mode of
the ADC input pins is set using two resistors connected to VCM.
The amplifier outputs can also be dc-coupled. Using the output common-mode control of the THS4509, the ADC
input pins can be biased to 1.5 V. In this case, use +4-V and – 1-V supplies for the THS4509 to ensure it's output
common-mode voltage (1.5 V) is at mid-supply.
Figure 69. Drive Circuit using THS4509
Refer to the EVM User Guide (SLAU196 ) for more information.
To ensure a low-noise common-mode reference, the VCM pin is filtered with a 0.1- μ F low-inductance capacitor
connected to ground. The VCM pin is designed to directly drive the ADC inputs. The input stage of the ADC
sinks a common-mode current in the order of 155 μ A at 125 MSPS (per input pin). describes the dependency of
the common-mode current and the sampling frequency.
This equation helps to design the output capability and impedance of the CM driving circuit accordingly.
Copyright © 2007, Texas Instruments Incorporated 43
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