LTC2415-1IGN datasheet(34/40 Pages) LINER

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LTC2415/LTC2415-1

sn2415 24151fs

APPLICATIO S I FOR ATIO

Correlated Double Sampling with the

LTC2415/LTC2415-1

Figure 37 shows the LTC2415/LTC2415-1 in a correlated

double sampling circuit that achieves a noise floor of

under 100nV. In this scheme, the polarity of the bridge is

alternated every other sample and the result is the average

of a pair of samples of opposite sign. This technique has

the benefit of canceling any fixed DC error components in

the bridge, amplifiers and the converter, as these will

alternate in polarity relative to the signal. Offset voltages

and currents, thermocouple voltages at junctions of dis-

similar metals and the lower frequency components of 1/f

noise are virtually eliminated.

The LTC2415/LTC2415-1 have the virtue of being able to

digitize an input voltage that is outside the range defined

by the reference, thereby providing a simple means to

implement a ratiometric example of correlated double

sampling.

This circuit uses a bipolar amplifier (LT1219—U1 and U2)

that has neither the lowest noise nor the highest gain. It

does, however, have an output stage that can effectively

suppress the conversion spikes from the LTC2415/

LTC2415-1. The LT1219 is a C-Load

TM stable amplifier

that, by design, needs at least 0.1

µF output capacitance to

remain stable. The 0.1

µF ceramic capacitors at the out-

puts (C1 and C2) should be placed and routed to minimize

lead inductance or their effectiveness in preventing enve-

lope detection in the input stage will be reduced. Alterna-

tively, several smaller capacitors could be placed so that

lead inductance is further reduced. This is a consideration

because the frequency content of the conversion spikes

extends to 50MHz or more. The output impedance of

most op amps increases dramatically with frequency but

the effective output impedance of the LT1219 remains

low, determined by the ESR and inductance of the capaci-

tors above 10MHz. The conversion spikes that remain at

the output of other bipolar amplifiers pass through the

feedback network and often overdrive the input of the

amplifier, producing envelope detection. RFI may also be

present on the signal lines from the bridge; C3 and C4

provide RFI suppression at the signal input, as well as

suppressing transient voltages during bridge commuta-

tion.

The wideband noise density of the LT1219 is 33nV

√Hz,

seemingly much noisier than the lowest noise amplifiers.

However, in the region just below the 1/f corner that is not

well suppressed by the correlated double sampling, the

average noise density is similar to the noise density of

many low noise amplifiers. If the amplifier is rolled off

below about 1500Hz, the total noise bandwidth is deter-

mined by the converter’s Sinc4 filter at about 12Hz. The

use of correlated double sampling involves averaging

even numbers of samples; hence, in this situation, two

samples would be averaged to give an input-referred

noise level of about 100nVRMS.

Level shift transistors Q4 and Q5 are included to allow

excitation voltages up to the maximum recommended for

the bridge. In the case shown, if a 10V supply is used, the

excitation voltage to the bridge is 8.5V and the outputs of

the bridge are above the supply rail of the ADC. U1 and U2

are also used to produce a level shift to bring the outputs

within the input range of the converter. This instrumenta-

tion amplifier topology does not require well-matched

resistors in order to produce good CMRR. However, the

use of R2 requires that R3 and R6 match well, as the

common mode gain is approximately –12dB. If the bridge

is composed of four equal 350

Ω resistors, the differential

component associated with mismatch of R3 and R6 is

nearly constant with either polarity of excitation and, as

with offset, its contribution is canceled.

C-Load is a trademark of Linear Technology Corporation.

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