sm72501mfx National Semiconductor Corporation, sm72501mfx Datasheet - Page 16

• Manufacturer Part Number: sm72501mfx
• Description: Solarmagic Precision, Cmos Input, Rrio, Wide Supply Range Amplifier
• Manufacturer: National Semiconductor Corporation

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PRECISION CURRENT SOURCE
The SM72501 can be used as a precision current source in
many different applications.
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
signal. Amplifier A2 is a buffer that eliminates the error current
from the load side of the R
feedback resistor if it were connected to the load side of the
R
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.
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/
ing N amplifiers in parallel as shown in
voltage noise on the output of this circuit is divided by the
square root of the number of amplifiers used in this parallel
S
resistor. In general, the circuit is stable as long as the
. This input voltage noise can be further reduced by plac-
FIGURE 6. Precision Current Source
S
Figure 6
resistor that would flow in the
shows a typical preci-
Figure
S
as the feedback
7. The total
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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
tion circuit. Typical resistor values are:
R
G
= 10Ω, R
shows a schematic of this input voltage noise reduc-
FIGURE 7. Noise Reduction Circuit
F
= 1 kΩ, and R
O
= 1 kΩ.
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