LTC1966CMS8#TRPBF Linear Technology, LTC1966CMS8#TRPBF Datasheet - Page 28

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

Specifications of LTC1966CMS8#TRPBF

• 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: Lead free / RoHS Compliant

LTC1966
Interfacing with an ADC
The LTC1966 output impedance and the RMS averaging
ripple need to be considered when using an analog-to-
digital converter (ADC) to digitize the LTC1966 RMS result.
The simplest configuration is to connect the LTC1966
directly to the input of a type 7106/7136 ADC as shown
in Figure 25a. These devices are designed specifically for
DVM/DPM use and include display drivers for a 3 1/2 digit
LCD segmented display. Using a dual slope conversion,
the input is sampled over a long integration window, which
results in rejection of line frequency ripple when integration
time is an integer number of line cycles. Finally, these parts
have an input impedance in the GΩ range, with specified
input leakage of 10pA to 20pA. Such a leakage, combined
with the LTC1966 output impedance, results in just 1µV
to 2µV of additional output offset voltage.
applicaTions inForMaTion
Another type of ADC that has inherent rejection of RMS
averaging ripple is an oversampling ∆∑. With most, but not
all, of these devices, it is possible to connect the LTC1966
output directly to the converter input. Issues to look out
28
LTC1966
OUT RTN
OUTPUT
Figure 25a. Interfacing to DVM/DPM ADC
Figure 25b. Interfacing to LTC2420
5
6
LTC1966
OUT RTN
OUTPUT
C
AVE
1V
5
6
2
3
4
V
V
GND
LTC2420
REF
IN
C
AVE
1966 F25b
SDO
SCK
CS
31
30
7106 TYPE
IN HI
IN LO
SERIAL
DATA
DIGITALLY CORRECT
LOADING ERRORS
1966 F25a
for are the input impedance, and any input sampling cur-
rents. The input sampling currents drawn by ∆∑ ADCs
often have large spikes of current with short durations
that can confuse some op amps, but with the large C
needed by the LTC1966 these are not an issue.
The average current is important, as it can create LTC1966
errors; if it is constant it will create an offset, while aver-
age currents that change with the voltage level create gain
errors. Some converters run continuously, others only
sample upon demand, and this will change the results in
ways that need to be understood. The LTC1966 output
impedance has a loose tolerance relative to the usual re-
sistors and the same can be true for the input impedance
of ∆∑ ADC, resulting in gain errors from part-to-part. The
system calibration techniques described in the following
section should be used in applications that demand tight
tolerances.
One example of driving an oversampling ∆∑ ADC is shown
in Figure 25b. In this circuit, the LTC2420 is used with a
1V V
1V, and the LTC2420 has a ±12.5% extended input range,
this configuration matches the two ranges with room to
spare. The LTC2420 has an input impedance of 16.6MΩ,
resulting in a gain error of –0.4% to –0.6%. In fact, the
LTC2420 DC input current is 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 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 nominal 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.
REF
. Since the LTC1966 output voltage range is about
1966fb
AVE