Datasheet
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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)
REFERENCE
ADS6245 , ADS6244
ADS6243 , ADS6242
SLAS542A – MAY 2007 – REVISED JULY 2007
Figure 87 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 87 ). As shown in the figure, R
IN
helps to isolate the
amplifier output from the switching inputs of the ADC. Together with C
IN
, 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
voltage of the ADC input spins 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 1.5V. In this case, use +4 V and -1 V supplies for the THS4509 to ensure its output
common-mode voltage (1.5V) is at mid-supply.
Figure 87. 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). Equation 1 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.
The ADS624X has built-in internal references REFP and REFM, requiring no external components. Design
schemes are used to linearize the converter load seen by the references; this and the on-chip integration of the
requisite reference capacitors eliminates the need for external decoupling. The full-scale input range of the
converter can be controlled in the external reference mode as explained below. The internal or external reference
modes can be selected by programming the register bit <REF> (Table 14 ).
Copyright © 2007, Texas Instruments Incorporated Submit Documentation Feedback 45
Product Folder Link(s): ADS6245 ADS6244 ADS6243 ADS6242