lmp7701max National Semiconductor Corporation, lmp7701max Datasheet - Page 17

• Manufacturer Part Number: lmp7701max
• Description: Precision, Cmos Input, Rrio, Wide Supply Range Amplifiers
• Manufacturer: National Semiconductor Corporation

PRECISION CURRENT SOURCE
The LMP7701/LMP7702/LMP7704 can each be used as a
precision current source in many different applications. Figure
6 shows a typical precision current source. This circuit imple-
ments a precision voltage controlled current source. Amplifier
A1 is a differential amplifier that uses the voltage drop across
R
nates the error current from the load side of the R
that would flow in the feedback resistor if it were connected
to the load side of the R
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.
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 LMP7701/LMP7702/LMP7704 have the very low input
voltage noise of 9 nV/
further reduced by placing N amplifiers in parallel as shown
in Figure 7. The total voltage noise on the output of this circuit
S
as the feedback signal. Amplifier A2 is a buffer that elimi-
FIGURE 6. Precision Current Source
S
. This input voltage noise can be
resistor. In general, the circuit is
S
resistor
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is divided by the square root of the number of amplifiers used
in this parallel combination. This is because each individual
amplifier acts as an independent noise source, and the aver-
age noise of independent 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
FIGURE 7. Noise Reduction Circuit
F
= 1 kΩ, and R
O
= 1 kΩ.
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