AD7937

–7–

REV. 0

APPLICATION HINTS

Output Offset: CMOS D/A converters in circuits such as Fig-

ures 4 and 5 exhibit a code-dependent output resistance which

in turn can cause a code-dependent error voltage at the output

of the amplifier. The maximum amplitude of this error, which

(25

10

Suitable op amps are the AD711C and its dual version, the

AD712C. These op amps have a wide bandwidth and high slew

rate and are recommended for wide bandwidth ac applications.

AD711/AD712 settling time to 0.01% is typically 3

µs.

Temperature Coefficients: The gain temperature coefficient

of the AD7937 has a maximum value of 5 ppm/

°C and typical

value of 1 ppm/

°C. This corresponds to worst case gain shifts of

2 LSBs and 0.4 LSBs respectively over a 100

°C temperature range.

When trim resistors R1 (R3) and R2 (R4) are used to adjust full

scale range as in Figure 4, the temperature coefficient of R1 (R3)

and R2 (R4) should also be taken into account.

High Frequency Considerations: AD7937 output capacitance

works in conjunction with the amplifier feedback resistance to

add a pole to the open-loop response. This can cause ringing or

oscillation. Stability can be restored by adding a phase compen-

sation capacitor in parallel with the feedback resistor. This is

shown as C1 and C2 in Figures 4 and 5.

Feedthrough: The dynamic performance of the AD7937 depends

upon the gain and phase stability of the output amplifier, together

with the optimum choice of PC board layout and decoupling

components.

MICROPROCESSOR INTERFACING

The byte loading structure of the AD7937 makes it very easy to

interface the device to any 8-bit microprocessor system. Figure

8 shows an example 8-bit interface between the AD7937 and a

generic 8-bit microcontroller/DSP processor. Pins D7 to D0 of

the processor are connected to pins D7 to D0 of the AD7937.

When writing to the DACs, the lower 8 bits must be written

first, followed by the upper four bits. The upper four bits should

be output on data lines D0 to D3.

ADDRESS

DECODE

AD7937*

D7

D0

CS

UPD

WR

D7

D0

R/

W

DATA

BUS

CONTROLLER/

DSP PROCESSOR

CLR

FROM

SYSTEM

RESET

A0

A0

A1

A1

UPPER BITS

OF ADDRESS

BUS

*

Figure 8. AD7937 8-Bit Interface