IEEE Solid-State Circuits Magazine - Fall 2017 - 96

When designing readout circuits for physiological
signal measurement, one needs to understand
that applications define the required signal
quality and the circuit architecture.
dependent. High input impedance is
easy to achieve (usually limited by the
parasitic capacitance, Cp of the input
buffer). Moreover, the noise is mainly
limited by buffers and R1. One of the
design tradeoffs is between power
consumption and noise. To achieve
large dynamic range at the input
stage, either large currents through
the resistor (meaning more power
consumption) or a large resistor (meaning more noise) is required. It is,
however, superior to the 3-op-amp IA
from power and CMRR (nothing to do
with mismatch of the resistors) perspective. Still, other performances,
such as offset, 1/f noise, and CMRR,
need to be further improved by apply-

ing a compensation circuit technique,
which is addressed in Figure 6(b) [6].
With the help of chopping, not only
can mismatches of input buffers
be compensated but also significant
improvements in low-frequency noise
(1/f) can be achieved.

High-Pass Filter Implementation
A chopper amplifier is basically a dccoupled amplifier that won't be able to
achieve input dc blocking. Figure 6(b)
shows the concept of the dc servo
loop having the same effect as an accoupled input for a chopper amplifier.
The purpose of the dc servo loop is to
sense the output dc level and regulate
the input of the amplifier such that

R

C

C

VIN+

VIN+
R2/R1

VO

VIN-

VIN-
C

R

R

R

R

R2/R1

VO

C

R

Classical HPF

Floating HPF

CM Input Impedance = R
High-Pass Cut Off = 1/2πRC

CM Input Impedance = ∞
High-Pass Cut Off = 1/2πRC

FIGURE 7: A passive high-pass filter.

IBIO

IBIO

ZT

ZBIO

V

ZT

ZBIO

ZT

ZT

IBIO

IBIO

Voltage Excitation

Current Excitation

IBIO = V/(2 × ZT + ZBIO)

IBIO = IEXCITED

FIGURE 8: Voltage versus current excitation.

96

ISOURCE

FA L L 2 0 17

IEEE SOLID-STATE CIRCUITS MAGAZINE

ISINK

the dc component of the input signal
can be properly removed. When the
negative feedback loop contains an
extremely large time-constant, lowpass filter, the overall response ends
up with high-pass filter characteristics
near dc once the loop settles.
Instead of a dc servo loop, a passive HPF can be utilized before the IA.
Fig ure 7 presents the circuit implementation for ac-coupling input
to the IA with a cutoff frequency
of 1/2rRC. The classical HPF suffers from finite common-mode input
impedance that may degrade the CMRR.
Alternatively, the floating HPF has very
large common-mode input impedance
by eliminating resistors to ground
while still having the same cutoff frequency. Notice that the floating HPF is
very useful to sustain high CMRR of
the system at the cost of using several
discrete passive components.

Bioimpedance Measurement
Electrical current excitation and voltage signal readout (or vice versa)
through electrodes lead bioimpedance measurement possible. This is
an emerging research area that has
received more and more attention as
bioimpedance is an important parameter to analyze body composition.
One way to implement a current signal
is to utilize a voltage source through
electrodes (voltage excitation), and
the other method is to directly inject
a current signal through the current
source (current excitation), as shown
in Figure 8. For the voltage excitation,
the electrode impedance Z T is added
to the bioimpedance Z BIO leading the
current through the tissue (I BIO) contains error. Alternatively, the current
excitation is more accurate as the
amount of current through the tissue (I BIO) identical to the I SOURCE/SINK .
However, voltage across electrode impedance challenges current source
design, and usually high supply voltage is required to overcome this.
A bipolar or tetrapolar electrode
system is utilized to measure electrode-tissue impedance (ETI) and
bioimpedance (Bio-Z), respectively.
As shown in Figure 9, the ETI can be
Table of Contents for the Digital Edition of IEEE Solid-State Circuits Magazine - Fall 2017
