Circuit Description

The LM9040 is fabricated in CMOS technology and is de-

signed to operate from a single well regulated 5V supply

The IC consists of two independent differential amplifiers

which are designed using two-phased switched capacitor

networks (SCN) The differential inputs have a common

mode operating range of 2V above and below ground The

SCN includes the input sampling the lowpass filter cold

sensor bias voltage and the gain circuitry Each amplifier

has an independent voltage comparator to detect an open

inverting input pin Additional support circuitry includes the

TLH12372 – 13

FIGURE 3 Simplified Circuit

Oscillator

The device contains an internal oscillator which is used to

drive the internal two-phase clock generator The oscillator

requires an external resistor value of 178 kX from the

‘‘OSC

mines the charge rate of the internal capacitor and thus

sets the oscillator frequency The internal oscillator capaci-

tor is matched to the switched capacitor networks so that

the absolute capacitance values are not as important as is

the absolute ratios of the capacitors The oscillator frequen-

cy is approximately 200 kHz

The oscillator resistor should be located as close to the

OSC

RES pin as possible Any variation of the oscillator

resistor value any stray capacitance on the OSC

RES pin

or any changes in the supply voltage will result in a change

in the oscillator frequency This will directly affect the device

Differential Input Impedance and Low Pass filter response

Additional circuitry takes the oscillator signal and generates

two non-overlapping clock signals and a CLK

OUT signal

The clock signals operate at one half the oscillator frequen-

cy or typically 100 kHz This results in a Nyquist frequency

of typically 50 kHz

Clock OutClock In

For the input stage to work with common mode voltages

needed The CLK

OUT pin is used to provide the AC sig-

an external coupling capacitor A minimum coupling capaci-

tor value of 100 pF to a maximum value of 01 mF is recom-

mended The CLK

erator circuitry

Differential Input Circuit

The input stage can be best described as a switched Sam-

ple and Difference circuit (see

Figure 4 ) When the input

charged by an amount equal to the common mode voltage

ential input voltage and a proportional charge will be trans-

ferred through the virtual ground via the gain stage

TLH12372 – 14

FIGURE 4 Simplified Switched Capacitor Input Circuit

4