MAX532
Dual, Serial-Input,
Voltage-Output, 12-Bit MDAC
______________________________________________________________________________________ 13
DACA
AGNDA
VOUTA
MAX532
RFBA
R/2
3
2
VREFA
1
V
IN
4
R
2
4096
CODE
V
OUT
V
IN
=
-4096
CODE
V
OUT
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:
V
OUT
/V
IN
= -REQA/RFBA,
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:
V
OUT
-4096
——— = ———
V
IN
CODE
The code may be programmed between 1 and (2
12
-1).
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.
