By Jim Harrison

Many contemporary applications, including data acquisition,
industrial control, and medical diagnostics, require linear measurements with both
accuracy and speed. The LTC2508-32 provides a unique combination of 14-bit
resolution at 1-Ms/s speed and 32-bit resolution at 61 s/s to 3.9 ks/s. The IC
simultaneously provides two output codes: a 32-bit digitally filtered high-precision
low-noise code, and a 22-bit no-latency composite code.


The 32-bit A/D yields integral linearity error performance
of ±0.5 ppm typical, ±3.5 ppm maximum, and uses a configurable digital filter
with a low noise output. It is specified with a zero-scale error of ±13 ppm and
a full-scale error of ±100 ppm. The configurable digital filter reduces
measurement noise by low-pass filtering and down-sampling the stream of data
from the SAR A/D. This converter’s dynamic range is 145 db with a down-sampling
factor of 16384 and 131 dB with a DF of 256. Total harmonic distortion is
typically −118 dB.

The fast side output is in the form of a 14-bit differential
+ 8-bit common-mode word output at 1 Ms/s with no latency. It has an integral linearity
error of ±0.1 LSB differential and ±0.1 LSB common mode. The common-mode measurement
is useful in many sensor interfaces to detect abnormal conditions.

The chip operates from a 2.5-V supply, has guaranteed operation
to 85°C (LTC2508I), and takes only 24 mW at 1 Ms/s. It comes in a 7 × 4 plastic
DFN24 package and requires an external 2.5 to 5-V reference. The device has a
CMRR of 128 db. Pricing for the chip starts at $8.95 ea/1,000.