Evaluation of single-bit sigma-delta modulator DAC for electrical impedance spectroscopy

Abstract

In this paper, single-bit sigma-delta modulator based digital-analog conversion (DAC) is evaluated as potential driver in multichannel electrical impedance spectroscopy (EIS) arrays. The sinusoidal excitation with typical frequency range of a few Hz to a few M Hz is a challenging task, which only very few state of the art implementations are even able to realize on chip, where the corresponding DAC consumes major amounts of area and power. Sigma-delta modulators offer signal generation with 1-bit representation, which could be advantageously multiplexed, distributed, and used on switches to generate analog output; thus its use could potentially lower the area and power consumption. This paper investigates the use of a single-bit sigma-delta DAC as direct driver for EIS experiments. Performances and limitations of the proposed system are experimentally analyzed and discussed. EIS experiments of micro-electrodes could be successfully conducted in a range from 20 Hz to 1 M Hz and matched with reference measurements to better than 95 % accuracy.

An Evaluation Study of Various Excitation Signals for Electrical Impedance Spectroscopy

“…This architecture is chosen for two reasons. First, excitation voltages are typically in the range of few tens of mV for safety and biocompatibiltiy reasons [7]. Thus, utilizing a voltage-output DAC would mean only few least significant bits (LSB) of the DAC would be used to generate the signal, which translates to a small signal to noise ratio (SNR).…”

“…This is is also known as phase sensitive analog lock-in detection [5]. Alternatively, digital lock-in detection can be performed which eliminates the limitations of the analog lock-in detection [7]. In this method the response of the SUT is digitized and then a fast Fourier transform (FFT) is applied.…”

“…Digital signal excitation is a very attractive choice due to their higher immunity to noise, easy multiplexing, easy implementation of the 2 reference voltages, etc. [7]. A sigmadelta modulator (Σ∆M) DAC consists of two parts, a digital modulator and an analog low-pass filter, which removes the high frequency quantization noise.…”

“…In contrast, in galvanic measurements [4], SUT is excited with current sources and the response is recorded in the voltage domain. Excitation signals can have different forms such as single-tone frequency [5], multi-tone [6], pseudo random noise [5], and sigma-delta modulated [7]. These signals can be divided to two main categorizes of analog (multi-valued) and digital (binary) excitation signals.…”

“…On the other hand, digital signal sources can be realized by use of global bitstream modualtion, such as a sigma delta modulator (Σ∆M) DAC or linear feedback shift register (LFSR), and only two local switches at the multi-electrode array can perform the acutal 1-bit D/A conversion. [7], [5]. Due to digital characteristics of the signals, they are immune from noise and they can be easily distributed and multiplexed between multi-electrode arrays.…”