AD7845_15 Datasheet, PDF (8/12 Pages) Analog Devices – LC MOS Complete 12-Bit Multiplying DAC

AD7845
APPLICATIONS 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
1111
1000
0100
0010
0001
0000
0000
0000
0000
1111
0000
0000
0000
0000
1000
0100
0010
0001
1111
0000
0000
0000
0000
0000
0000
0000
0000
Gain
4096/4095 ≈ 1
2
4
8
16
32
64
128
256
Error (%)
0.04
0.07
0.13
0.26
0.51
1.02
2.0
4.0
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 = I₃ = I₂ + I₁
I₁ =
D
× |V_IN
R_DAC
|
,

 ^D
=
0
to
4095
4096


1
I₂ = _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
–8–
REV. B