SM72501 Datasheet, PDF (17/28 Pages) National Semiconductor (TI) – SolarMagic Precision, CMOS Input, RRIO, Wide Supply Range Amplifier

SM72501
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SNIS157C – JANUARY 2011 – REVISED APRIL 2013
R
V₁
R
V⁺
-
A₁
+
V^-
R
R
V₂
R_S
V⁺
-
A₂
+
V^-
I = (V₂ ± V₁)
R_S
Z LOAD
Figure 46. Precision Current Source
The equation for output current can be derived as follows:
V₂R (V0 ± IRS)R
V₁R
V₀R
+
=
+
R+R
R+R
R+R R+R
(4)
Solving for the current I results in the following equation:
I = ^{V2 ± V1}
R_S
(5)
LOW INPUT VOLTAGE NOISE
The SM72501 has a very low input voltage noise of 9 nV/ . This input voltage noise can be further reduced by
placing N amplifiers in parallel as shown in Figure 47. 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:
1
REDUCED INPUT VOLTAGE NOISE =
N
e_n²₁+e²_n2+
+e²_nN
=1
N
Ne²_n =
N
N
e_n
= ¹ en
N
(6)
Figure 47 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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