Datasheet

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-VCC
1 Fm
+VCC
1 Fm
+2.5V
V
I
300 W
THS4031
680pF
15 W
VIN
1 Fm
10kW
0
300 W
R24
R20
C5
R5
C18
U4
R13
C3
R25
C20
3.2 Reference
Analog Interface
The type of input capacitors used in the signal path can make a few decibels of difference in AC
performance. The factory recommends using either polypropylene or C0G type capacitor in the input
signal path. The polypropylene capacitor causes the least distortion of the input signal and has excellent
long-term stability, but is bulky and expensive. The C0G ceramic is lower cost, comes in smaller
packages, and performs well in many applications. C0G capacitors however, tend to be less stable over
time and temperature when compared polypropylene capacitors. The 680-pF capacitors installed on the
EVM are low cost C0G type manufactured by TDK Corporation.
Figure 1. Input Buffer Circuit
The ADS7866, ADS7867, and ADS7868 reference voltage is derived from the supply voltage internally.
Consequently, the supply voltage to these converters should be driven with a low impedance source and
be decoupled to ground at the chip. It is recommended that, at least, a 1- µ F and 10-nF decoupling
capacitors be placed close to the chip. The traces from these capacitors to the VDD pin should be wide,
low impedance traces.
The converters themselves draw very little current from the supply lines. Therefore, the supply voltage pin
for the ADS7866, ADS7867, and ADS7868 can be connected directly to the system supply or to a low
noise and low drift reference chip. The EVM provides users the option to power the A/D converter from
either the analog supply voltage applied at TP15 or from an onboard 2.5-V reference chip.
SLAU181 – July 2006 ADS7866EVM/ADS7867EVM/ADS7868EVM 3
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