Datasheet
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
155 mAxFs
125 MSPS
ADS6445, ADS6444
ADS6443, ADS6442
SLAS531B –MAY 2007–REVISED DECEMBER 2009
www.ti.com
Using Differential Amplifier Drive Circuits
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 (10 dB in Figure 87). 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
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 1.5 V. In this case, use +4 V and -1 V supplies for the THS4509 to ensure that it's
output common-mode voltage (1.5 V) is at mid-supply.
Figure 87. Drive Circuit using THS4509
Refer to the EVM User Guide (SLAU196) for more information.
INPUT COMMON MODE
To ensure a low-noise common-mode reference, the VCM pin is filtered with a 0.1-μF 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 μA at 125 MSPS (per input pin). Equation 1 describes the
dependency of the common-mode current and the sampling frequency.
(1)
This equation helps to design the output capability and impedance of the CM driving circuit accordingly.
REFERENCE
The ADS644X 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> (refer to Table 13).
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