Datasheet
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APPLICATION INFORMATION
60
60
16 pF
20
5 pF
V
DD
IN
GND
Driving the VIN and V
DD
Pins of the ADS7887 and ADS7888
V
DD
VIN
GND
CS
SDO
SCLK
10 nF1 F
V
DD
V
DD
VIN
GND
CS
SDO
SCLK
10 nF1 F
GND
IN
OUT
3 V
1 F
5 V
REF3030
ADS7887
ADS7888
SLAS468 – JUNE 2005
Figure 36. Typical Equivalent Sampling Circuit
The VIN input to the ADS7887 and ADS7888 should be driven with a low impedance source. In most cases
additional buffers are not required. In cases where the source impedance exceeds 200 Ω , using a buffer would
help achieve the rated performance of the converter. The THS4031 is a good choice for the driver amplifier
buffer.
The reference voltage for the ADS7887 and ADS7888 A/D converters are derived from the supply voltage
internally. The devices offer limited low-pass filtering functionality on-chip. The supply to these converters should
be driven with a low impedance source and should be decoupled to the ground. A 1-µF storage capacitor and a
10-nF decoupling capacitor should be placed close to the device. Wide, low impedance traces should be used to
connect the capacitor to the pins of the device. The ADS7887 and ADS7888 draw very little current from the
supply lines. The supply line can be driven by either:
• Directly from the system supply.
• A reference output from a low drift and low drop out reference voltage generator like REF3030 or REF3130.
The ADS7887 and ADS7888 can operate off a wide range of supply voltages. The actual choice of the
reference voltage generator would depend upon the system. Figure 38 shows one possible application
circuit.
• A low-pass filtered version of the system supply followed by a buffer like the zero-drift OPA735 can also be
used in cases where the system power supply is noisy. Care should be taken to ensure that the voltage at
the V
DD
input does not exceed 7 V (especially during power up) to avoid damage to the converter. This can
be done easily using single supply CMOS amplifiers like the OPA735. Figure 39 shows one possible
application circuit.
Figure 37. Supply/Reference Decoupling Capacitors
Figure 38. Using the REF3030 Reference
18