LTC1968 datasheet(22/28 Pages) LINER

22 / 28 page

LTC1968

1968f

not zero at 0V, but rather at one half its reference, so both

an output offset and a gain error will result. These errors

will vary from part to part, but with a specific LTC1968 and

LTC2420 combination, the errors will be fixed, varying less

than ±0.05% over temperature. So a system that has digi-

tal calibration can be quite accurate despite the nominal

gain and offset error. With 20 bits of resolution, this part

is more accurate than the LTC1968, but the extra resolu-

tion is helpful because it reduces nonlinearity at the LSB

transitions as a digital gain correction is made. Further-

more, its small size and ease of use make it attractive.

This connection is shown in Figure 21b, where the LTC2420

is set to continuously convert by grounding the CS pin. The

gain error will be less if CS is driven at a slower rate,

however, the rate should either be consistent or at a rate

low enough that the LTC1968 and its output capacitor have

fully settled by the beginning of each conversion, so that

the loading errors are consistent.

Other types of ADCs sample the input signal once and

perform a conversion on that one sample. With these

ADCs (Nyquist ADCs), a post filter will be needed in most

cases to reduce the peak error with low input frequencies.

The DC-accurate filter of Figure 13 is attractive from an

error standpoint, but it increases the impedance at the ADC

input. In most cases, the buffered post filter of Figure 12

will be more appropriate for use with Nyquist analog-to-

digital converters.

SYSTEM CALIBRATION

The LTC1968 static accuracy can be improved with end-

system calibration. Traditionally, calibration has been

done at the factory, or at a service depot only, typically

using manually adjusted potentiometers. Increasingly,

systems are being designed for electronic calibration

where the accuracy corrections are implemented in digital

code wherever possible, and with calibration DACs where

necessary. Additionally, many systems are now designed

for self calibration, in which the calibration occurs inside

the machine, automatically without user intervention.

Whatever calibration scheme is used, the linearity of the

LTC1968 will improve the calibrated accuracy over that

achievable with older log/antilog RMS-to-DC converters.

Additionally, calibration using DC reference voltages are

essentially as accurate with the LTC1968 as those using

AC reference voltages. Older log/antilog RMS-to-DC con-

verters required nonlinear input stages (rectifiers) whose

linearity would typically render DC-based calibration

unworkable.

The following are four suggested calibration methods.

Implementations of the suggested adjustments are de-

pendent on the system design, but in many cases, gain and

output offset can be corrected in the digital domain, and

will include the effect of all gains and offsets from the

LTC1968 output through the ADC. Input offset voltage, on

the other hand, will have to be corrected with adjustment

to the actual analog input to the LTC1968.

AC-Only, 1 Point

The dominant error at full scale will be caused by the gain

error, and by applying a full-scale sine wave input, this

error can be measured and corrected for. Unlike older log/

antilog RMS-to-DC converters, the correction should be

made for zero error at full scale to minimize errors through-

out the dynamic range.

The best frequency for the calibration signal is roughly ten

times the –0.1% DC error frequency. For 10µF, –0.1% DC

error occurs at 6Hz, so 60Hz is a good calibration frequency,

although anywhere from 60Hz to 100Hz should suffice.

APPLICATIO S I FOR ATIO

Figure 21a. Interfacing to DVM/DPM ADC

Figure 21b. Interfacing to LTC2420

CAVE

LTC1968

OUTPUT

OUT RTN

7106 TYPE

IN HI

IN LO

1968 F21a

CAVE

LTC1968

OUTPUT

OUT RTN

LTC2420

VIN

SERIAL

DATA

DIGITALLY CORRECT

LOADING ERRORS

GND

SDO

SCK

1968 F21b

LTC1968 датащи(PDF) 22 Page - Linear Technology

LTC1968 датащи(HTML) 22 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

датащи скачать

ссылки URL