Datasheet
3.2 Reference
Analog Interface
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The 15- Ω series resistor works with the capacitor, C20, to filter the input signal and isolates the amplifier
from the capacitive load. The 680-pF capacitor to ground at the ADC input works with the series resistor to
filter the input signal, behaving as a charge reservoir. It provides a short to ground for high-frequency
noise and kickback currents when the device switches from hold to sample mode. This external filter
capacitor also works with the amplifier to charge the internal sampling capacitor during sampling mode.
The type of input capacitors used in the signal path can make a few decibels of difference in ac
performance. TI recommends using either a 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 type is lower cost, comes in smaller packages, and
perform well in many applications, but tends to be less stable over time and temperature compared to
polypropylene capacitors. The 680-pF capacitors installed on the EVM are low-cost, C0G-type capacitors
manufactured by TDK Corporation.
The ADS788x reference voltage is derived internally from the supply voltage. Consequently, the supply
voltage to these converters must be driven with a low-impedance source and be decoupled to ground at
the chip. It is recommended that, at least, a pair of 1- µ F and 10-nF decoupling capacitors be placed close
to the chip. Use wide, low-impedance traces from these capacitors to the VDD pin.
The converters themselves draw very little current from the supply lines. Therefore, the supply voltage pin
for the ADS788x 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 ADC from either the analog supply voltage applied
at TP4 or from an onboard 2.5-V reference chip.
Table 3. Jumper Setting
(1)
Jumper Setting
Reference
Designator Description 1-2 2-3
Set negative supply of U2 to ground. Installed
(2)
Not installed
SJP1
Select negative supply of U2 to –VCC. Not installed Installed
Set negative supply of U4 to ground. Installed Not installed
SJP2
Select negative supply of U4 to –VCC. Not installed Installed
(2)
Apply CS from P2.1 to the ADC chip select pin Installed Not installed
W1
Apply FS from P2.7 to the ADC chip select pin Not installed Installed
(2)
Set BVDD to +5VD Installed Not installed
W2
Set BVDD to +3.3VD Not installed Installed
(2)
Set 2.5 V to W4 (pin 3) Installed
(2)
Not installed
W3
Set user applied voltage to W4 (pin 3) Not installed Installed
Set DUT power supply pin to 5 V Installed
(2)
Not installed
W4
Set DUT power supply pin to voltage on W3 (pin 2). Not installed Installed
(1)
These jumper settings are for revision B of the ADS788xEVM printed circuit board (PCB).
(2)
Factory-installed.
8 ADS788xEVM SLAU166B – October 2005 – Revised August 2008
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