LTC1966 datasheet(25/32 Pages) LINER

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LTC1966

sn1966 1966fas

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

will vary from part to part, but with a specific LTC1966 and

LTC2420 combination, the errors will be fixed, varying

less than

±0.05% over temperature. So a system that has

digital calibration can be quite accurate despite the nomi-

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

part is more accurate than the LTC1966, but the extra

resolution is helpful because it reduces nonlinearity at the

LSB transitions as a digital gain correction is made.

Furthermore, its small size and ease of use make it

attractive.

This connection is shown in Figure 22b, 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 LTC1966 and its output capacitor

have fully settled by the beginning of each conversion, so

that the loading errors are consistent.

The low power consumption of the LTC1966 makes it well-

suited for battery-powered applications, and its slow

output (DC) makes it an ideal candidate for a micropower

ADC. Figure 10 in Application Note 75, for instance, details

a 10-bit ADC with a 35ms conversion time that uses just

µA of supply current. Such an ADC may also be of use

within a 4mA to 20mA loop.

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 14 is attractive from an

error standpoint, but it increases the impedance at the

ADC input. In most cases, the buffered post filter of

Figure 13 will be more appropriate for use with Nyquist

analog-to-digital converters.

SYSTEM CALIBRATION

The LTC1966 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

LTC1966 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 LTC1966 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

LTC1966 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 LTC1966.

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

throughout the dynamic range.

APPLICATIO S I FOR ATIO

CAVE

LTC1966

OUTPUT

OUT RTN

7106 TYPE

IN HI

IN LO

1966 F22a

CAVE

LTC1966

OUTPUT

OUT RTN

LTC2420

VIN

SERIAL

DATA

DIGITALLY CORRECT

LOADING ERRORS

GND

SDO

SCK

1966 F22b

Figure 22a. Interfacing to DVM/DPM ADC

Figure 22b. Interfacing to LTC2420

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