AD7846

Rev. G | Page 17 of 24

DIGITAL FEEDTHROUGH

In the preceding interface configurations, most digital inputs to

the AD7846 are directly connected to the microprocessor bus.

Even when the device is not selected, these inputs are constantly

changing. The high frequency logic activity on the bus can feed

through the DAC package capacitance to show up as noise on

the analog output. To minimize this digital feedthrough, isolate

the DAC from the noise source. Figure 29 shows an interface

circuit that isolates the DAC from the bus.

Note that to make use of the AD7846 readback feature using

the isolation technique of Figure 29, the latch needs to be

bidirectional.

AD7846*

+5V

DATA BUS

CS

LDAC

CLR

MICRO-

PROCESSOR

A1 TO A15

ADDRESS

DECODE

ADDRESS BUS

D0 TO D15

DB0 TO DB15

R/W

R/W

2×

74LS245

B BUS

A BUS

DIR

G

*LINEAR CIRCUITRY OMITTED FOR CLARITY

Figure 29. AD7846 Interface Circuit Using Latches to Minimize Digital Feedthrough