Intan Technologies, LLC: low-noise amplifier microchips for electrophysiology, EKG, EMG, EEG, ECoG, and neural recording headstages and bio instrumentation

Intan Technologies RHA2132 32-channel amplifier chip in plastic QFN package and in bare die form.

Traditional electrophysiology recording system
Intan Technologies amplifier chips can replace many of the large, bulky analog front-end amplifier units traditionally used in electrophysiology monitoring systems.

Close up of traditional electrophysiology amplifier
Like this traditional amplifier, Intan amplifiers have configurable lower and upper cutoff frequencies, so their bandwidth can be optimized for particular signals of interest.

The Intan Technologies RHA2116 16-channel integrated amplifier array requires only three external resistors (to set amplifier bandwidth), two capacitors (to smooth power supply), and one square centimeter of board area.

Introduction to Intan Technologies amplifier chips

Intan Technologies multi-channel amplifier chips are small, low-power silicon devices that replace the following modules found in traditional electrophysiological recording systems:

Preamplifiers

Intan amplifiers have high input impedances (more than 10 megohms at 1 kHz), so high-impedance electrodes may be connected directly to the chips. The amplifiers use proprietary low-noise circuits to achieve an input-referred RMS noise level of 2 microvolts, allowing small physiological signals to be resolved.

High-pass filters

The tiny ac signals acquired by physiological electrodes are usually accompanied by relatively large dc offsets that drift slowly with time. Intan amplifiers incorporate high-pass filters on each channel to block large dc potentials and undesirable low-frequency drift or artifacts. The cutoff frequency of these filters is configurable by means of a single off-chip resistor.

Differential amplifiers

Biopotential signals must be amplified (relative to a reference electrode or negative electrode) from the microvolt level to the volt level to match the full input range of high-resolution A/D converters. Intan amplifiers provide suitable gain for observing signals up to +/-5 mV in amplitude with sub-microvolt resolution using modern A/D converters. Differential amplifiers should also have good rejection of common-mode noise.

Low-pass filters

Prior to sampling and digitization, a low-pass filter must be used to limit the bandwidth of each amplifier and prevent aliasing. Additionally, this filter may be used to attenuate undesirable high-frequency signals or artifacts. Intan amplifiers incorporate 3rd-order Butterworth low-pass filters on each channel. The cutoff frequency of these filters is configurable using two off-chip resistors.

Analog multiplexer

Intan chips contain many amplifier channels to facilitate device miniaturization. The signals from an array of amplifiers are multiplexed to a single output pin so that all channels can share a single A/D converter. Simple digital control lines are used to sequence the selection of channels. This allows many electrodes to be accessed using a small number of control and data lines.

Electrode impedance measurement

Intan chips facilitate the in situ measurement of electrode impedances by allowing an ac test current to be routed to any selected electrode. The resulting voltage amplitude (measured using the on-chip amplifiers) can be used to calculate impedance.

Basic Circuit Example

A basic schematic for a 16-channel recording system using an Intan Technologies RHA2116 amplifier array is shown below. Three external resistors are used to select the desired cutoff frequencies of the on-chip high- and low-pass filters. Two small capacitors help to smooth out the power supply near the chip. Two digital control signals are used to cycle through the amplifier channels. The entire system consumes only one square centimeter of board area (see photo at left). The 32-channel RHA2132 chip allows even higher amplifier density.

Basic amplifier schematic

Typical electrophysiology recording system diagram

The diagram below shows the modules of a typical electrophysiology recording system. Mouse over the amplifier stages for more information. The orange dashed box encompasses all functions that are performed by an Intan amplifier chip.

Unipolar vs. bipolar electrodes

The diagram above depicts a system with unipolar recording electrodes and a common reference electrode. The RHA2116 and RHA2132 16- and 32-channel amplifier chips support this architecture. For some applications (e.g., external EMG), it may be desirable to use independent bipolar electrodes for each amplifier channel to enhance the rejection of common-mode interference. The RHA2216 16-channel amplifier chip supports this architecture. See the RHA2000 series product page for more information and a complete datasheet.

Modern multi-electrode arrays require large numbers
of wires, bulky cables, and unwieldy connectors. By
time-multiplexing many amplifier channels, cables and
connectors can be made much smaller and lighter.

Signals obtained from high-impedance electrodes are
susceptible to corruption from interference (e.g., 50/60 Hz
line noise). Signals actively driven by amplifiers have a
low impedance and are less susceptible to noise pickup.

To take full advantage of high-resolution A/D converters,
electrophysiological signals must be amplified from
the microvolt range to the volt range.

Modern analog-to-digital converters can operate at
sampling rates of more than 1 MHz, so many amplifier
channels can be shared by using an analog MUX.
Typical full-scale input voltage ranges are around 2 V,
so signals must be amplified to this level to take full
advantage of the A/D converter resolution .

A fast analog multiplexer (MUX) allows many amplifiers
to share a single A/D converter. The output of the MUX
must settle quickly after a new channel is selected so that
an accurate high-resolution sample is obtained.

Low-pass filters must be configured to less than half
of the A/D converter sampling rate to prevent aliasing,
and may also be used to block undesired high-frequency
signals or artifacts.

High gain differential amplifiers are used to boost the
signals to larger voltage levels required by A/D converters
and to provide a voltage reference for all signals so that
common-mode noise can be rejected.

High-pass filters must be used to remove the large dc offsets
present at the electrode-tissue interface, along with any undesired
low-frequency signals (e.g., microphonic movement artifacts).

The size and power requirements of conventional amplifiers
and filters has traditionally required them to be located some
distance from the headstage and electrodes. While the signals
on the preamp cable are driven with a low impedance, they
remain small in amplitude and susceptible to corruption.

Low-noise preamplifiers or 'headstages' are used in some systems
to buffer the signals from the electrodes (i.e., lower the signal impedance).
The gain of these preamps must be low due to the presence of large dc
offsets from the electrode-tissue interface that could saturate the preamps.

Microvolt-level signals from high-impedance electrodes are
susceptible to noise and interference coupling to the electrode
cable through electric or magnetic fields. Preamplifiers should
be placed as close as possible to the electrodes to guard against
noise pickup.

Tiny, weak ac signals obtained from electrodes
must be amplified to higher amplitudes suitable for
digitization. Large dc offsets are often present
at the electrode-tissue interface.