REDUCED INPUT VOLTAGE NOISE =

2

2

2

=

1

N

1

N

2

=

N

N

=

1

N

I =

R + R

+

=

+

R + R

R + R

R + R

+

Z

LOAD

R

R

R

R

V

+

V

+

V

-

V

-

+

-

-

LMP7701, LMP7702, LMP7704

www.ti.com

SNOSAI9H – SEPTEMBER 2005 – REVISED MARCH 2013

PRECISION CURRENT SOURCE

The LMP7701/LMP7702/LMP7704 can each be used as a precision current source in many different

applications. Figure 50 shows a typical precision current source. This circuit implements a precision voltage

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.

Figure 50. Precision Current Source

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/

√Hz. This input voltage noise

can be further reduced by placing N amplifiers in parallel as shown in Figure 51. 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 combination.

This is because each individual amplifier acts as an independent noise source, and the average 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 51 shows a schematic of this input voltage noise reduction circuit. Typical resistor values are:

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