LMP8358MA NSC [National Semiconductor], LMP8358MA Datasheet - Page 29

• Manufacturer Part Number: LMP8358MA
• Description: Zero-Drift, Programmable Instrumentation Amplifier with Diagnostics
• Manufacturer: NSC [National Semiconductor]

START UP AND POWER ON RESET
During power on, 50µs after V
the internal register to 0000x. If the digital supplies and inputs
are undefined after the Power On Reset transients could oc-
cur which can cause erroneous data to be written over the
default values in the register. The following should be done to
prevent this from happening:
• Bring all supplies up at the same time. All power supplies,
analog and digital, should be brought up together within 40µs
so that the supplies are not undefined after the Power On
Reset at 50µs. This is easiest done by tying the VHSER/VL-
PAR and VLSER/VHPAR pins to the analog supplies. —
Parallel Mode
• Immediately after power on, write to the register the value
needed for the application. (This is always recommended.) —
Serial Mode
LAYOUT
The LMP8358 is a precision device that contains both analog
and digital sections as shown in the Block Diagram. The PCB
should be carefully designed to minimize the interaction be-
tween the analog and digital sections and to maximize the
performance of the part. This should include the following:
0.1µF ceramic capacitors should be placed as close as pos-
sible to each supply pin. If a digital supply pin is tied to an
analog pin only one 0.1µF capacitor is needed for both pins.
A larger 1µF or 10µF capacitor should be located near the part
for each supply.
Digital and analog traces should be kept away from each oth-
er. Analog and digital traces should not run next to each other,
if they do the digital signal can couple onto the analog line.
The LMP8358 pinout is set up to simplify layout by not having
analog, power, and digital pins mixed together. Pins 1 — 6
are the analog signals, pins 7 — 10 are the power pins, and
pins 11 — 14 are the digital signals.
Be aware of the signal and power return paths. The return
paths of the analog, digital, and power sections should not
cross each other and the return path should be underneath
the respective signal or power path. The best PCB layout is if
the bottom plane of the PCB is a solid plane.
The REFF and REFS pins are connected to the bottom side
of the gain resistors of the LMP8358 as shown in the Block
Diagram. Any impedance on these pins will change the spec-
ified gain. If the REFF and REFS pins are to be connected to
ground they should be tied directly to the ground plane and
not through thin traces that can add impedance. If the REFF
and REFS pins are to be connected to a voltage, the voltage
source must be low impedance. This can be done by adding
an op amp, such as the LMP7701, set up in a buffer configu-
+
− V
−
> 1V the LMP8358 resets
29
ration with the LMP7701 output connected to REFF, the
negative input of the op amp connected to REFS, and the
desired reference voltage connected to the positive input of
the op amp as shown in
DIFFERENTIAL BRIDGE SENSOR
Non-amplified differential bridge sensors, which are used in
a variety of applications, typically have a very small differential
output signal. This small signal needs to be accurately am-
plified before it can be used by an ADC.
The high DC performance of the LMP8358 makes it a good
choice for use with a differential bridge sensor. This perfor-
mance includes low input offset voltage, low input offset volt-
age drift, and high CMRR. The on chip EMI rejection filters
available on the LMP8358 help remove the EMI interference
introduced to the signal as shown in
the overall system performance.
The circuit in
ical bridge sensor using the LMP8358. The typical output
voltage of a resistive load cell is 2mV/V. If the bridge sensor
is using a 5V supply the maximum output voltage will be 2mV/
V × 5V = 10mV. The bridge voltage in this example is the
same as the LMP8358 and ADC161S626 supply voltage of
+5V. This 10mV signal must be accurately amplified by the
LMP8358 to best match the dynamic range of the ADC. This
is done by setting the gain of the LMP8358 to 200 which will
give an output from the LMP8358 of 2V. To use the complete
range of the ADC161S626 the V
to half of the input or 1V. This is done by the resistor divider
on the V
the ADC and the REFF and REFS pins of the LMP8358 can
be set to +2.5V to set the signal at the center of the supply. A
resistor divider supplies +2.5V to the positive input of an
LMP7701 set up in a buffer configuration. The LMP7701 acts
as a low impedance source for the REFF pin. The V
VHSER/VLPAR pins should all be set to the same voltage as
the microcontroller, +3.3V in this example. The VLSER/VH-
PAR pin should be connected to ground. The resistor and
capacitor between the LMP8358 and the ADC161S626 serve
a dual purpose. The capacitor is a charge reservoir for the
sampling capacitor of the ADC. The resistor provides isolation
for the LMP8358 from the capacitive load. The values listed
in the ADC161S626 datasheet are 180Ω for the resistor and
the 470pF for the capacitor. These two components also form
a low pass filter of about 1.9MHz. If a filter is needed to at-
tenuate disturbance from the internal auto−zeroing at 12kHz
and the ping−pong frequency at 50kHz of the LMP8358 these
values could be changed to 7870Ω and 0.01µF which will
make a filter with a corner of about 2kHz.
REF
pin of the ADC161S626. The negative input of
Figure 14
shows a signal path solution for a typ-
Figure
14.
REF
of the ADC should be set
Figure 14
and improves
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