DELIVERING A BIOMEDICAL
ALL-ROUNDER
Delivering one main signal chain is a critical requirement,
when it comes to wearable mulitparameter monitoring
devices. Jan-Hein Broeders explains
Oxygen saturation,
electrocardiography, blood
pressure, and respiration rates
are all key parameters that need to be
measured and monitored closely. With
an ageing population and the growing
cost of healthcare, access to this type
of medical monitoring equipment,
outside of a hospital, is now becoming
more important.
Multiparameter monitors need
to be accurate, small, and able to
operate for a long period of time from
a single battery charge.
Many such systems look to
combine two or more measurements
and each one is performed by a
dedicated analogue front end,
resulting in the need for several chips,
each with its own analogue-to-digital
converter (ADC), its own interface to
the main processor, and several power
supplies and reference voltages that
need to be decoupled.
As a result, there are many
redundant building blocks, which is
certainly not ideal from a size and
power perspective. In a wearable
system, the aim is usually to have
one main signal chain to which each
sensor can be connected – but a new
family of biomedical front ends is
looking to address this.
The ADPD4000 has been designed
around two identical receive channels,
which can be sampled simultaneously.
Each is differentially built, which
makes it possible to measure any
sensor input in either a single-ended
or differential measuring mode.
The input stage is a
transimpedance amplifier, with
programmable gain, followed by
a band-pass filter and integrator,
capable of integrating 7.5 pC per
sample.
The ADC is a 14-bit successive
approximation register (SAR) converter
with a maximum sampling rate of 1
MSPS. In front of each of the signal
chains is an 8-channel multiplexer that
gives the front end flexibility in routing
the various sensor signals into the
AFE.
Various signals can be measured
and it is possible to modify the AFE
as an optical front end, for instance,
to measure either optical heart rate or
oxygen saturation. In this mode, the
photocurrent is measured, so a high
transimpedance input stage is needed
to convert current into a voltage.
Interference, coming from ambient
light, also needs to be cancelled.
Next to a receive signal chain, the
chip supports eight output drivers,
which can be used to provide stimuli.
Each can configure one or more
outputs to drive LEDs for an optical
measurement, or one or more driver
outputs can be used as excitation
for impedance measurement,
either to measure skin impedance
or electrode impedance - which
affects the measurement quality
- while performing a biopotential
measurement.
Figure 1: High level
block diagram of the
ADPD4000 family
The chip has been designed to
allow the user to pre-program each
configuration or measurement in
a certain time slot – up to 12 time
slots are supported - which makes
the system easy to use once it is
configured. Additionally, the chip
does not require additional processor
resources, keeping the system’s
overall power consumption at a
minimum.
On the chip, oversampling improves
the effective number of bits (ENOB)
of the ADC. The decimated data
path is 32 bits wide. Measurement
results can be stored in a 256-byte
or 512-byte deep FIFO (ADPD400x vs.
ADPD410x).
There is an integrated time stamp
function to support synchronisation
among data samples, which is
required once multiple sensor data
are used to find correlation among the
16 28 April 2020 www.newelectronics.co.uk
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