LTC2440IGN datasheet(23/28 Pages) LINER

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LTC2440

sn2440, 2440fas

Figure 17. Direct Connection to Low Impedance (<350

Ω) Source

is Possible if the Sensor is Located Close to the ADC.

Direct Connection to Low Impedance Sources

If the ADC can be located physically close to the sensor, it

can be directly connected to sensors or other sources with

impedances up to 350

Ω with no other components re-

quired (see Figure 17).

Buffering the LTC2440

Many applications will require buffering, particularly where

high impedance sources are involved or where the device

being measured is located some distance from the

LTC2440. When buffering the LTC2440 a few simple steps

should be followed.

Figure 19 shows a network suitable for coupling the inputs

of a LTC2440 to a LTC2051 chopper-stabilized op amp.

The 3

µV offset and low noise of the LTC2051 make it a

good choice for buffering the LTC2440. Many other op

amps will work, with varying performance characteristics.

The LTC2051 is configured to be able to drive the 1

µF

capacitors at the inputs of the LTC2440. The 1

µF capaci-

tors should be located close to the ADC input pins.

The measured total unadjusted error of Figure 19 is well

within the specifications of the LTC2440 by itself. Most

autozero amplifiers will degrade the overall resolution to

some degree because of the extremely low input noise of

the LTC2440, however the LTC2051 is a good general

purpose buffer. The measured input referred noise of two

LTC2051s buffering both LTC2440 inputs is approxi-

mately double that of the LTC2440 by itself, which reduces

the effective resolution by 1-bit for all oversample ratios.

Adding gain to the LTC2051 will increase gain and offset

errors and will not appreciably increase the overall resolu-

tion, so it has limited benefit.

Procedure For Coupling Any Amplifier to the LTC2440

The LTC2051 is suitable for a wide range of DC and low

frequency measurement applications. If another amplifier

is to be selected, a general procedure for evaluating the

suitability of an amplifier for use with the LTC2440 is

suggested here:

1. Perform a thorough error and noise analysis on the

amplifier and gain setting components to verify that the

amplifier will perform as intended.

2. Measure the large signal response of the overall circuit.

The capacitive load may affect the maximum slew rate of

the amplifier. Verify that the slew rate is adequate for the

APPLICATIO S I FOR ATIO

Longer Connections to Low Impedance Sources

If longer lead lengths are unavoidable, adding an input

capacitor close to the ADC input pins will average the

charging pulses and prevent reflections or ringing (see

Figure 18). Averaging the current pulses results in a DC

input current that should be taken into account. The

resulting 110k

Ω input impedance will result in a gain error

of 0.44% for a 350

Ω bridge (within the full scale specs of

many bridges) and a very low 12.6ppm error for a 2

Ω

thermocouple connection.

Figure 18. Input Capacitors Allow Longer Connection Between

the Low Impedance Source and the ADC.

2440 F17

REF+

REF–

IN+

IN–

LTC2440

µF

4.5V to 5.5V

2440 F18

µF

4.5V to 5.5V

REMOTE

THERMOCOUPLE

VREF+ VCC

GND

IN+

IN–

LTC2440

µF

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