LMP8358

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SNOSB09B – APRIL 2010 – REVISED MARCH 2013

APPLICATION INFORMATION

INTRODUCTION

The LMP8358 is a precision programmable gain instrumentation amplifier. Its gain can be programmed to 10, 20,

50, 100, 200, 500 or 1000 through an SPI-compatible serial interface or through a parallel interface. Alternatively,

its gain can be set to an arbitrary value using external resistors. Note that at low gains the dynamic range is

limited by the maximum input differential voltage of ±100mV. The LMP8358 uses patented techniques to

measure and continuously correct its input offset voltage, eliminating offset drift over time and temperature, and

the effect of 1/f noise. Its ground sensing CMOS input features a high CMRR and low input bias currents. It is

capable of sensing differential input voltages in a common-mode range that extends from 100 mV below the

negative supply to 1.4V below the positive supply, making it an ideal solution for interfacing with ground-

referenced sensors, supply-referenced sensor bridges, and any other application requiring precision and long

term stability. Additionally, the LMP8358 includes fault detection circuitry, so open and shorted inputs can be

detected, as well a deteriorating connection to the signal source. Other features that make the LMP8358 a

versatile solution for many applications are: its rail-to-rail output, low input voltage noise and high gain-bandwidth

product.

TRANSIENT RESPONSE TO FAST INPUTS

The LMP8358 is a current-feedback instrumentation amplifier that consists of two auto-zeroed input stages.

These two input stages are operated in a ping-pong fashion: as one stage is auto-zeroed the other stage

provides the path between the input pins and the output. The auto-zeroing decreases offset, offset drift, and 1/f

noise while the ping-pong architecture provides a continuous path between the input and the output. As with all

devices that use auto-zeroing, care must be taken with the signal frequency used with the device. On-chip

continuous auto-zero correction circuitry eliminates the 1/f noise and significantly reduces the offset voltage and

offset voltage drift; all of which are very low-frequency events. For slow-changing sensor signals, below 2kHz,

this correction is transparent. Higher-frequency signals as well as fast changing edges will show a settling and

ramping time lasting about 1

μs. Like all auto-zeroing devices, if the input frequency is above the auto-zero

frequency, aliasing will occur. This can occur both at the auto-zeroing frequency of about 12kHz and the ping-

pong frequency of about 50kHz. If needed, a low-pass filter should be placed on the output of the LMP8358 to

filter out this disturbance.

COMMUNICATION WITH THE PART AND REGISTER DESCRIPTION

The LMP8358 supports a serial and a parallel digital interface mode as shown in Figure 47 and Figure 48.

Parallel user mode Gain is set using G0, G1 and G2 pins. The shutdown mode can be activated by asserting

SHDN. Fault detection features are unavailable.

Serial user mode The part is SPI-programmable through SDI, SCK, SDO and CSB. All features are available.

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