ADM4073
Rev. A | Page 10 of 12
THEORY OF OPERATION
The current from the source flows through R
SENSE
, which gen-
erates a voltage drop, V
SENSE
, across the RS+ and RS− terminals
of the sense amplifier. The Input Stage Amplifier A1 regulates
its inputs to be equal, thereby shunting a current proportional
to V
SENSE
/R
G1
to the output current mirror. This current is then
multiplied by a gain factor of b in the output stage current mir-
ror and flows through R
GD
to generate V
OUT
. Therefore, V
OUT
is
related to V
SENSE
by the ratio of R
G1
to R
GD
and the current gain
of b.
V
OUT
= A
V
× V
SENSE
where:
A
V
= R
GD
/R
G1
× b
A
v
is equal to different voltages depending upon the model of
the device.
• 20 V/V for ADM4073T.
• 50 V/V for ADM4073F.
• 100 V/V for ADM4073H.
2V TO 28V
3V TO 28V
0.1µF
V
OUT
R
SENSE
I
LOAD
I
RG1
V
OUT
RS+
GND
AV
RS–
V
CC
R
G2
R
GD
= 12kΩ
R
G1
ADM4073
I
RGD
CURRENT
MIRROR
05131-026
OUT
Figure 25. Functional Block Diagram
R
SENSE
The ADM4073 has the ability to sense a wide variety of currents
by selecting a particular sense resistor. Select a suitable output
voltage for full-scale current, such as 10 V for 10 A. Then, select
a gain model that gives the most efficient use of the sense volt-
age range (150 mV max).
In the example above, using the ADM4073H (gain of 100) gives
an output voltage of 10 V when the sense voltage is 100 mV.
Use the following equation to determine what value of sense
resistor gives 100 mV with 10 A flowing through it:
R
SENSE
= 100 mV/10 A
R
SENSE
= 10 mΩ
To measure lower currents accurately, use as large a sense
resistor as possible to utilize the higher end of the sense voltage
range. This reduces the effects of the offset voltage errors in the
internal amplifier.
When currents are very large, it is important to take the I
2
R
power losses across the sense resistor into account. If the sense
resistor’s rated power dissipation is not sufficient, its value can
drift, giving an inaccurate output voltage or it could fail alto-
gether. This, in turn, causes the voltage across the RS+ and RS−
pins to exceed the absolute maximum ratings.
If the monitored supply rail has a large amplitude high
frequency component, choose a sense resistor with low
inductance.
INPUT COPPER PCB TRACE OUTPUT
3V TO 28
0.1µF
R
SENSE
OUT
RS+
GND
RS–
V
CC
ADM4073
05131-004
Figure 26. Using PCB Trace for Current Sensing
OUTPUT (OUT)
The output stage of the ADM4073 is a current source driving a
pull-down resistance. To ensure optimum accuracy, care must
be taken not to load this output externally. To minimize output
errors, ensure OUT is connected to a high impedance input
stage. If this is not possible, output buffering is recommended.
The percent error introduced by output loading is determined
with the following formula:
LOAD
INTOUT
LOAD
RRRError +
_
/1100%
where:
R
LOAD
is the external load applied to OUT.
R
OUT_INT
is the internal output resistance (12 kΩ).
V
OUT
= (I
LOAD
× R
SENSE
) × A
V
