Figure 16 shows the AD7837/AD7847 connected for bipolar
operation. The coding is offset binary as shown in Table IV.
When VIN is an ac signal, the circuit performs 4-quadrant multi-
plication. To maintain the gain error specifications, resistors R1,
R2 and R3 should be ratio matched to 0.01%. Note that on the
AD7847 the feedback resistor RFB is internally connected to
Figure 16. Bipolar Offset Binary Operation
Table IV. Bipolar Code Table
DAC Latch Contents
Analog Output, VOUT
1111 1111 1111
1000 0000 0001
1000 0000 0000
0111 1111 1111
0000 0000 0000
Note 1 LSB = V IN .
PROGRAMMABLE GAIN AMPLIFIER (PGA)
The dual DAC/amplifier combination along with access to RFB
make the AD7837 ideal as a programmable gain amplifier. In this
application, the DAC functions as a programmable resistor in the
amplifier feedback loop. This type of configuration is shown
in Figure 17 and is suitable for ac gain control. The circuit con-
sists of two PGAs in series. Use of a dual configuration provides
greater accuracy over a wider dynamic range than a single PGA
solution. The overall system gain is the product of the individual
gain stages. The effective gains for each stage are controlled by
the DAC codes. As the code decreases, the effective DAC
resistance increases, and so the gain also increases.
Figure 17. Dual PGA Circuit
The transfer function is given by
VOUT = REQA × REQB
VIN RFBA RFBB
where REQA, REQB are the effective DAC resistances controlled
by the digital input code:
where RIN is the DAC input resistance and is equal to RFB and
N = DAC input code in decimal.
The transfer function in (1) thus simplifies to
VOUT = 212 × 212
VIN N A NB
where NA = DAC A input code in decimal and NB = DAC B
input code in decimal.
NA, NB may be programmed between 1 and (212–1). The zero
code is not allowed as it results in an open loop amplifier
response. To minimize errors, the digital codes NA and NB
should be chosen to be equal to or as close as possible to each
other to achieve the required gain.