"Chapter 1 - The Op Amp's Place in the World" - HTL Wien 10

The simultaneous equations are given below.
VREF
Figure 18–13. DAC Current Sink to Actuator Interface Circuit
+V
1 m b
4 2 b
RF
_
+
RF
+V
–V
DAC to Actuator Analog Interface
VO
Designing Low-Voltage Op Amp Circuits
(18–30)
(18–31)
From these equations we find that b = –2 and m = –3. The slope and intercept values are
substituted into Equation 18–28 to get Equation 18–32.
V (18–32)
OUT –I (–m) IN b mIIN b
The current equation for the circuit shown in Figure 18–13 is given below as Equation
18–33, and after algebraic manipulation it becomes Equation 18–34.
V OUT
R F
I IN V REF
R G
V OUT I IN R F V REF
RF RG (18–33)
(18–34)
Comparing terms between Equations 18–29 and 18–34 enables the extraction of m and
b.
R (18–35)
F |m| 3
|b| V REF
RF RG 2
5
RF RG (18–36)
(18–37)
These equations are written in terms of mA and kΩ. R G is selected as 51 kΩ, so R F =
20 kΩ. When I IN = 2 mA, the compliance of the DAC is 0.0 V.
18-19