Datasheet
REDUCED INPUT VOLTAGE NOISE =
e
n1
+e
n2
+
2 2
+e
nN
2
=
1
N
1
N
Ne
n
2
=
N
N
e
n
=
1
e
n
N
I =
V
2
± V
1
R
S
V
2
R
R + R
(V
0
± IR
S
)R
V
1
R V
0
R
+
= +
R + R R + R
R + R
+
R
S
Z
LOAD
V
1
V
2
R
R
R
R
V
+
V
+
V
-
V
-
+
-
-
I = (V
2
± V
1
)
R
S
A
1
A
2
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
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 general, the circuit is
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:
R
G
= 10Ω, R
F
= 1 kΩ, and R
O
= 1 kΩ.
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