Dual, Serial-Input,

Voltage-Output, 12-Bit MDAC

______________________________________________________________________________________

13

DACA

AGNDA

VOUTA

MAX532

RFBA

R/2

3

2

VREFA

1

4

R

2

4096

CODE

=

-4096

CODE

Figure 12. Programmable-Gain Amplifer

Programmable-Gain Amplifier (PGA)

The DAC/amplifier combination, along with access to

the feedback resistors, makes the MAX532 ideal as a

programmable-gain amplifier. In this application, the

DAC functions as a programmable resistor in the feed-

back loop. This type of configuration is shown in Figure

12, and is suitable for AC gain control. The DAC code

controls the gain for the PGA. As the code decreases,

the effective DAC resistance increases, and so the gain

also increases. The transfer function is given by:

where RFBA is the value of the feedback resistor (R/2),

and REQA is the effective DAC resistance controlled

by the digital input code:

R

4096

REQA = —— (————) ,

2

CODE

where CODE is the DAC code in decimal.

The transfer function is thus:

-4096

——— =

———

CODE

The zero code is not allowed, as it results in an open-

loop amplifier response.

Power-On Reset

On power-up, the internal DAC latches are set to

00 . . . . .00.