Texas Instruments LMP8601 User Manual
LMP8601
Application Note 1940 LMP8601 Evaluation Board User's Manual
Literature Number: SNOA539
LMP8601 Evaluation Board
User's Manual
National Semiconductor
Application Note 1940
BeB Hennink
February 6, 2009
Introduction
The LMP8601 Current Sense evaluation Board is designed
to evaluate the LMP8601, 60V Common Mode, Bidirectional
Precision Current Sensing Amplifier. This board has a
LMP8601 part mounted on the PCB together with the required
de-coupling capacitor, power supply connections, Input and
Output connectors and a 3 pin header for connecting the offset pin to either GND or VS.
30089701
FIGURE 1. The LMP8601 Evaluation Board
General Description
The LMP8601 and LMP8601Q are fixed 20x gain precision
amplifiers. The part amplifies and filters small differential signals in the presence of high common mode voltages. The
input common mode voltage range is −22V to +60V when
operating from a single 5V supply. With 3.3V supply, the input
common mode voltage range is from −4V to +27V. The
LMP8601 and LMP8601Q are members of the Linear Monolitic Precision (LMP®) family and are ideal parts for unidirectional and bidirectional current sensing applications. All
parameter values of the part that are shown in the tables of
the LMP8601 data sheet are 100% tested and all bold values
are also 100% tested over temperature.
The part has a precise gain of 20x which is adequate in most
targeted applications to drive an ADC to its full scale value.
The fixed gain is achieved in two separate stages, a preamplifier with a gain of 10x and an output stage buffer amplifier
with a gain of 2x. The connection between the two stages of
the signal path is brought out on two pins to enable the possibility to create an additional filter network around the output
buffer amplifier. These pins can also be used for alternative
configurations with different gain as described in the applications section of the data sheet .
The mid-rail offset adjustment pin enables the user to use
these devices for bidirectional single supply voltage current
sensing. The output signal is bidirectional and mid-rail referenced when this pin is connected to the positive supply rail.
With the offset pin connected to ground, the output signal is
unidirectional and ground-referenced .
The LMP8601Q incorporates enhanced manufacturing and
support processes for the automotive market, including defect
detection methodologies. Reliability qualification is compliant
with the requirements and temperature grades defined in the
AEC Q100 standard
LMP8601 Operating Conditions
•
Temperature Range −40°C to 125°C
•
Power Supply Voltage 3.0V ≤ VS ≤ 5.5V
•
CMVR at VS = 3.3V −4V to 27V
•
CMVR at VS = 5.0V −22V to 60V
© 2009 National Semiconductor Corporation 300897 www.national.com
LMP8601 Evaluation Board User's Manual AN-1940
Description of the LMP8601
Evaluation Board
The LMP8601 evaluation board requires a power supply with
a voltage between 3.0V and 5.5V , the supply current will be
<2 mA in normal operation with high impedance load on the
output of the part. The positive supply voltage is connected to
VS (P5) The negative supply voltage is connected to GND
(P6).
The offset voltage on the output of the LMP8601 is determined by the voltage on the offset pin which is connected to
pin2 of J1 and the offset voltage will be V
Offset
/2. This pin is
normally connected with a jumper to GND or VS , but this pin
can also be driven from a low impedance source (<10 Ω).
With a jumper on J1 between 1–2 the offset pin is connected
to VS and the offset voltage is half the value of VS. In this
configuration the LMP8601 can be used for measuring bidirectional currents.
With a jumper on J1 between 2–3 the offset pin is connected
to GND and the output voltage is ground referenced. In this
configuration the LMP8601 is used for measuring unidirectional currents.
The input signal is connected between +IN (P1) and −IN (P2)
and the output signal is available at OUT (P3).
On the board there is the possibility to place a capacitor C2
which creates a low pass filter between the first and second
stage. This can be used to reduce the output noise and glitches that might appear from switching large common mode
voltages with very fast transients at the input . (Please see the
application section of the LMP8601 data sheet for more details about this filter).
the Header J2 is normally shorted with a jumper and connects
the first stage to the second stage . The application section of
the LMP8601 data sheet explains more about the alternative
configurations that can be made after removing the jumper on
J2.
30089704
FIGURE 2. Power Supply Connectors and Headers
PCB Layout Guidelines
This section provides general practical guidelines for PCB
layouts that use various power and ground traces. Designers
should note that these are only "rule-of-thumb" recommendations and the actual results are predicated on the final
layout.
DIFFERENTIAL SIGNALS
The two input pins of the LMP8601 form a differential pair that
must be handled following the rules given below.
•
Keep both signals coupled by routing them closely
together and keeping them of equal length.
•
Do not allow any other signal in between these two signals
of the differential pair.
•
Keep all impedances in both traces of the signal equal.
POWER, GROUND AND DE-COUPLING
Keep the power supply de-coupling capacitor close to the
power supply pin (VS of the part.)
Make sure all return currents of the signals can flow next to
the originating signals
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AN-1940
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