LTC1966CMS8 Linear Technology, LTC1966CMS8 Datasheet - Page 25

• Manufacturer Part Number: LTC1966CMS8
• Description: IC PREC RMS/DC CONV MCRPWR 8MSOP
• Manufacturer: Linear Technology

Specifications of LTC1966CMS8

• Current - Supply: 155µA
• Voltage - Supply: 2.7 V ~ 5.5 V
• Mounting Type: Surface Mount
• Package / Case: 8-MSOP, Micro8™, 8-uMAX, 8-uSOP,
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIO S I FOR ATIO
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
29 A 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.
LTC1966
OUT RTN
OUTPUT
Figure 22a. Interfacing to DVM/DPM ADC
Figure 22b. Interfacing to LTC2420
5
6
LTC1966
OUT RTN
OUTPUT
U
C
AVE
5
6
3
4
U
V
GND
LTC2420
IN
C
AVE
1966 F22b
SDO
SCK
CS
31
30
W
7106 TYPE
IN HI
IN LO
SERIAL
DATA
DIGITALLY CORRECT
LOADING ERRORS
1966 F22a
U
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.
LTC1966
sn1966 1966fas
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