SM72501 Datasheet, PDF (16/20 Pages) National Semiconductor (TI) – SolarMagic Precision, CMOS Input, RRIO, Wide Supply Range Amplifier

PRECISION CURRENT SOURCE
The SM72501 can be used as a precision current source in
many different applications. Figure 6 shows a typical preci-
sion current source. This circuit implements a precision volt-
age controlled current source. Amplifier A1 is a differential
amplifier that uses the voltage drop across R_S as the feedback
signal. Amplifier A2 is a buffer that eliminates the error current
from the load side of the R_S resistor that would flow in the
feedback resistor if it were connected to the load side of the
R_S resistor. In general, the circuit is stable as long as the
closed loop bandwidth of amplifier A2 is greater then the
closed loop bandwidth of amplifier A1. Note that if A1 and A2
are the same type of amplifiers, then the feedback around A1
will reduce its bandwidth compared to A2.
combination. This is because each individual amplifier acts as
an independent noise source, and the average noise of inde-
pendent sources is the quadrature sum of the independent
sources divided by the number of sources. For N identical
amplifiers, this means:
Figure 7 shows a schematic of this input voltage noise reduc-
tion circuit. Typical resistor values are:
R_G = 10Ω, R_F = 1 kΩ, and R_O = 1 kΩ.
FIGURE 6. Precision Current Source
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The equation for output current can be derived as follows:
Solving for the current I results in the following equation:
LOW INPUT VOLTAGE NOISE
The SM72501 has a very low input voltage noise of 9 nV/
. This input voltage noise can be further reduced by plac-
ing N amplifiers in parallel as shown in Figure 7. The total
voltage noise on the output of this circuit is divided by the
square root of the number of amplifiers used in this parallel
FIGURE 7. Noise Reduction Circuit
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