LMP7701_06 datasheet(17/22 Pages) NSC

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Application Information (Continued)

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

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 gen-

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/

. This input voltage noise can be

further reduced by placing 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 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:

G =10

Ω,R

F =1k

Ω, and R

O =1k

Ω.

20127305

FIGURE 6. Precision Current Source

20127356

FIGURE 7. Noise Reduction circuit

LMP7701 Datasheet(PDF) 17 Page - National Semiconductor (TI)

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