LMP8358 datasheet(29/32 Pages) NSC

29 / 32 page

START UP AND POWER ON RESET

During power on, 50µs after V+ − V− > 1V the LMP8358 resets

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-

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 Figure 14.

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 Figure 14 and improves

the overall system performance.

The circuit in Figure 14 shows a signal path solution for a typ-

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

REF of the ADC should be set

to half of the input or 1V. This is done by the resistor divider

on the V

REF pin of the ADC161S626. The negative input of

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

IOand

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 180

Ω 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 7870

Ω and 0.01µF which will

make a filter with a corner of about 2kHz.

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