11

®

ISO807

SIGNAL CONDITIONING

The FET switches used for the sample hold on many CMOS

A/D converters release a significant amount of charge injec-

tion which can cause the driving op amp to oscillate. The

amount of charge injection due to the sampling FET switch

on the ISO807 is approximately 5-10% of the amount on

similar ADCs with the charge redistribution DAC (CDAC)

architecture. There is also a resistive front end which attenu-

ates any charge which is released. The end result is a

FIGURE 4. Typical Transfer Functions With and Without External Resistors.

Digital Output

(b) Unipolar

–Full Scale

+Full Scale

Analog Input

Digital Output

(a) Bipolar

–Full Scale

+Full Scale

Analog Input

Typical Transfer Functions

With External Resistors

Typical Transfer Functions

Without External Resistors

minimal requirement for the drive capability on the signal

conditioning preceding the A/D. Any op amp sufficient for

the signal in an application will be sufficient to drive the

ISO807.

The resistive front end of the ISO807 also provides a

guaranteed

±25V overvoltage protection. In most cases, this

eliminates the need for external over voltage protection

circuitry.