LMP7701_06 NSC [National Semiconductor], LMP7701_06 Datasheet - Page 17

• Manufacturer Part Number: LMP7701_06
• Description: Precision, CMOS Input, RRIO, Wide Supply Range Amplifiers
• Manufacturer: NSC [National Semiconductor]

Application Information
PRECISION CURRENT SOURCE
The LMP7701/LMP7702/LMP7704 can each be used as a
precision current source in many different applications. Fig-
ure 6 shows a typical precision current source. This circuit
implements a precision voltage controlled current source.
Amplifier A1 is a differential amplifier that uses the voltage
drop across R
buffer that eliminates the error current from the load side of
the R
were connected to the load side of the R
eral, the circuit is stable as long as the closed loop band-
width of amplifier A2 is greater then the closed loop band-
width 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 very low input volt-
age 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
S
resistor that would flow in the feedback resistor if it
FIGURE 6. Precision Current Source
S
as the feedback signal. Amplifier A2 is a
. This input voltage noise can be
S
(Continued)
resistor. In gen-
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circuit is divided by the square root of the number of ampli-
fiers used in this parallel combination. This is because each
individual amplifier acts as an independent noise source,
and the average noise of independent sources is the quadra-
ture 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
reduction 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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