V
/
V
Rg
Gain =
k
5
:
+IN
-IN
OUT
V-
V+
RG
-
+
A
-
+
SOURCE
V
DMM1
DMM2
Rsns
+IN
-IN
OUT
V-
V+
RG
-
+
VCC
ohmic
LOAD
Rg1
Rg2
VCC
IN
LOAD
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Using the Evaluation Board
4
Using the Evaluation Board
4.1
Input and Output Signals
The evaluation board allows you to measure a bidirectional current, so the IN and LOAD banana
connectors can act either as the Source or LOAD pin. If the current flows from the IN pin to the LOAD pin,
a simple way to test the performance of the LMP8640 (LMP8645) is shown in
Figure 1
.
Figure 1. LMP8640 (LMP8645) Measurement Setup
The DMM1 is configured as a voltmeter, which measures the output of the LMP8640 (LMP8645), while the
DMM2 is configured as an Ammeter, which measures the current that flows in the LOAD. The source is a
voltage supply that makes sure to set a voltage in the range of -2 V to +46 V for LMP8640 (LMP8645) or -
2 V to +76 V for LMP8640HV (LMP8645HV).
4.2
Gain Selection of LMP8645
The evaluation board is provided with a shunt resistor Rsns (10 m
Ω
, @ 1%, 1W), while two gain resistors
Rg1 and Rg2 (10 k
Ω
, @1%) ensure a gain of 2 V/V for each current sense. The Gain is evaluated
according to formula in
Equation 3
:
(3)
The gain resistor must be chosen such that the max output voltage does not exceed the LMP8645 max
output voltage rating for a given common mode voltage (further details in the Datasheet).
4.3
Single Output
In the applications where a single measurement of a bidirectional current sense is needed is possible to
populate the evaluation board with two amplifiers and some resistors (see
Section 3.3
).
For instance, in the following configuration:
R1 = R2 = R3 = R4 = 10 k
Ω
,
R5 = R6 =10 k
Ω
OUT = V
REF
+(OUT
+
-OUT
-
)
V
REF
= 0.5*Vcc_Amp.
3
SNOA546C – November 2009 – Revised April 2013
AN-1975 LMP8640/LMP8645 Evaluation Board
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