ADS62C15_10 Datasheet, PDF (36/64 Pages) Texas Instruments – Dual Channel 11-Bits, 125 MSPS ADC With Parallel CMOS/DDR LVDS Outputs

ADS62C15
SLAS577D – JANUARY 2008 – REVISED JULY 2009....................................................................................................................................................... www.ti.com
0.1 mF
1:1
50 E
50 E
0.1 mF
50E
50E
1 :1
INP
INM
VCM
Figure 35. Drive Circuit at High Input Frequencies
Using Differential Amplifier Drive Circuits
Figure 36 shows a drive circuit using a differential amplifier (TI's THS4509) to convert a single-ended input to
differential output that can be interface to the ADC analog input pins. In addition to the single-ended to differential
conversion, the amplifier also provides gain (10 dB). RFIL helps to isolate the amplifier outputs from the switching
input of the ADC. Together with CFIL it also forms a low-pass filter that band-limits the noise (and signal) at the
ADC input. As the amplifier output is ac-coupled, the common-mode voltage of the ADC input pins is set using
two 200 Ω resistors connected to VCM.
The amplifier output 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 so that its output
common-mode voltage (1.5 V) is at mid-supply.
R_F
R_S
R_T
0.1 mF
+V_S
500 W
R_G
0.1 mF
0.1 mF ^{10 mF}
R_G
R_S || R_T
0.1 mF
500 W
-V_S
0.1 mF 10 mF
R_FIL
0.1 mF
5W
INP
C_FIL
200 W
C_FIL
R_FIL
0.1 mF
200 W
5W
INM
VCM
0.1 mF
R_F
Figure 36. Drive Circuit Using the THS4509
36
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