ADS5481 Datasheet, PDF (32/54 Pages) Texas Instruments – 16-Bit, 80/105/135-MSPS Analog-to-Digital Converters

ADS5517
SLWS203 – DECEMBER 2007
www.ti.com
Using 10-Ω series resistance and 25 Ω-3.3 pF-25 Ω as the R-C-R filter, high effective bandwidth (700 MHz) can
be achieved, (see Figure 35, transfer function from the analog input pins to the voltage across the sampling
capacitors).
In addition to the above ADC requirements, the drive circuit may have to be designed to provide a low insertion
loss over the desired frequency range and matched impedance to the source. For this, the ADC input impedance
has to be taken into account (Figure 36).
Example Drive Circuits
A suitable configuration using RF transformers and including the R-C-R filter is shown in Figure 34. Note the
15-Ω series resistors and the low common-mode impedance (using 33-Ω resistors terminated to VCM).
0.1 mF ^WBC1-1TLB
1:1
WBC1-1TLB
100 W
100 W
1:1
Z_i and TFADC
15 W
(Note A)
0.1 mF
33 W
33 W
25 W
3.3 pF
25 W
15 W
(Note A)
INP
INM
VCM
A. Use lower series resistance (≈ 5 Ω to 10 Ω) at high input frequencies (> 100 MHz)
Figure 34. Example Drive Circuit With RF Transformers
2
1
0
-1
-2
-3
-4
-5
-6
0
100 200 300 400 500 600 700 800 900 1000
f − Frequency − MHz
Figure 35. Analog Input Bandwidth, TFADC (Actual
Silicon Data)
500
450
400
350
300
250
200
150
100
50
0
0
100 200 300 400 500 600 700 800 900 1000
f − Frequency − MHz
Figure 36. Input Impedance, Z_I
32
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