Data Sheet
AD5940
GENERAL DESCRIPTION
The AD5940 is a high precision, low power analog front end (AFE)
designed for portable applications that require high precision,
electrochemical-based measurement techniques, such as amper-
ometric, voltammetric, or impedance measurements. The
AD5940 is designed for skin impedance and body impedance
measurements, and works with the AD8233 AFE in a complete
bioelectric or biopotential measurement system. The AD5940 is
designed for electrochemical toxic gas sensing.
The current inputs include two TIAs with programmable gain
and load resistors for measuring different sensor types. The first
TIA, referred to as the low power TIA, measures low bandwidth
signals. The second TIA, referred to as the high speed TIA,
measures high bandwidth signals up to 200 kHz.
An ultralow leakage, programmable switch matrix connects the
sensor to the internal analog excitation and measurement blocks.
This matrix provides an interface for connecting external RTIAs
and calibration resistors. The matrix can also be used to
multiplex multiple electronic measurement devices to the same
wearable electrodes.
The AD5940 consists of two high precision excitation loops
and one common measurement channel, which enables a wide
capability of measurements of the sensor under test. The first
excitation loop consists of an ultra low power, dual output string,
digital-to-analog converter (DAC), and a low power, low noise
potentiostat. One output of the DAC controls the noninverting
input of the potentiostat, and the other output controls the
noninverting input of the transimpedance amplifier (TIA). This
low power excitation loop is capable of generating signals from
dc to 200 Hz.
A precision 1.82 V and 2.5 V on-chip reference source is available.
The internal ADC and DAC circuits use this on-chip reference
source to ensure low drift performance for the 1.82 V and 2.5 V
peripherals.
The AD5940 measurement blocks can be controlled via direct
register writes through the serial peripheral interface (SPI)
interface, or, alternatively, by using a preprogrammable sequencer,
which provides autonomous control of the AFE chip. 6 kB of
static random access memory (SRAM) is partitioned for a deep
data first in, first out (FIFO) and command FIFO. Measurement
commands are stored in the command FIFO and measurement
results are stored in the data FIFO. A number of FIFO related
interrupts are available to indicate when the FIFO is full.
The second excitation loop consists of a 12-bit DAC, referred to
as the high speed DAC. This DAC is capable of generating high
frequency excitation signals up to 200 kHz.
The AD5940 measurement channel features a 16-bit, 800 kSPS,
multichannel successive approximation register (SAR) analog-
to-digital converter (ADC) with input buffers, a built in antialias
filter, and a programmable gain amplifier (PGA). An input mux
in front of the ADC allows the user to select an input channel
for measurement. These input channels include multiple
external current inputs, external voltage inputs, and internal
channels. The internal channels allow diagnostic measurements
of the internal supply voltages, die temperature, and reference
voltages.
A number of general-purpose inputs/outputs (GPIOs) are
available and are controlled using the AFE sequencer, which
allows cycle accurate control of multiple external sensor devices.
The AD5940 operates from a 2.8 V to 3.6 V supply and is
specified over a temperature range of −40°C to +85°C. The
AD5940 is packaged in a 56-lead, 3.6 mm × 4.2 mm WLCSP
package.
Rev. 0 | Page 5 of 130
ADI [ ADI ]
