Datasheet

AD7845
REV. B
–8–
APPLI CATION S CIRCUITS
PROGRAMMABLE GAIN AMPLIFIER (PGA)
The AD7845 performs a PGA function when connected as in
Figure 15. In this configuration, the R-2R ladder is connected
in the amplifier feedback loop. R
FB
is the amplifier input resis-
tor. As the code decreases, the R-2R ladder resistance increases
and so the gain increases.
V
OUT
= –V
IN
×
R
DAC
D
×
1
R
FB
,
D = 0 to
4095
4096






= –V
IN
×
R
DAC
D
×
1
R
DAC
=
–V
IN
D
, since R
FB
= R
DAC
Figure 15. AD7845 Connected as PGA
As the programmed gain increases, the error and noise also
increase. For this reason, the maximum gain should be limited
to 256. Table III shows gain versus code.
Note that instead of using R
FB
as the input resistor, it is also
possible to use combinations of the other application resistors,
R
A
, R
B
and R
C
. For instance, if R
B
is used instead of R
FB
, the
gain range for the same codes of Table II now goes from l/2
to 128.
Table III. Gain and Error vs. Input Code for Figure 15
Digital Inputs Gain Error (%)
1111 1111 1111 4096/4095 ≈ 10.04
1000 0000 0000 2 0.07
0100 0000 0000 4 0.13
0010 0000 0000 8 0.26
0001 0000 0000 16 0.51
0000 1000 0000 32 1.02
0000 0100 0000 64 2.0
0000 0010 0000 128 4.0
0000 0001 0000 256 8.0
PROGRAMMABLE CURRENT SOURCES
The AD7845 is ideal for designing programmable current
sources using a minimum of external components. Figures 16
and 17 are examples. The circuit of Figure 16 drives a program-
mable current I
L
into a load referenced to a negative supply.
Figure 17 shows the circuit for sinking a programmable current,
I
L
. The same set of circuit equations apply for both diagrams.
I
L
= I
3
= I
2
+ I
1
I
1
=
D ×|V
IN
|
R
DAC
,
D = 0 to
4095
4096






I
2
=
1
R1
D ×|V
IN
|
R
DAC






R
FB
=
D ×|V
IN
|
R1
, since R
FB
= R
DAC
I
L
=
D ×|V
IN
|
R1
+
D ×|V
IN
|
R
DAC
=
D ×|V
IN
|
R1
×
1 +
R1
R
DAC






Note that by making R1 much smaller than R
DAC
, the circuit
becomes insensitive to both the absolute value of R
DAC
and its
temperature variations. Now, the only resistor determining load
current I
L
is the sense resistor R1.
If R1 = 100 Ω, then the programming range is 0 mA to 100 mA,
and the resolution is 0.024 mA.
Figure 16. Programmable Current Source