# Minimally-Invasive Neural Interface for Distributed Wireless Electrocorticogram Recording Systems
## 5 Measurement Results
### System Characteristics Measurement
Figure 18 shows the measured variations of \(I_{REF}\) and \(V_{REF}\) as a function of input power supply voltage variations. It shows a linear sensitivity of 0.02%/V and 1.1%/V for voltage and current references, respectively, at the highest value. The output voltage and current references can be digitally controlled through 8 and 32 steps, respectively. The output current can be adjusted from 20 nA to 33 nA, and the output reference voltage from 0.71 V to 1.03 V. The measured reference current and voltage offset variations are 1.11 nA and 6.62 mV for 1\(\sigma\) variation, respectively. Figure 19 shows the process and temperature dependent variation of the reference current and voltage. The standard deviations of current and voltage among  
Figure 16: Measured SAR ADC characteristics: (**a**) INL/DNL and (**b**) FFT output spectra for 1.9474 kHz and 12.7621 kHz input signals at the sampling frequency of 31.25 kS/s. Figure 17: Measured SAR ADC characteristics: (**a**) SNDR/SFDR for various input and sampling frequency and (**b**) power consumption of the component blocks in the SAR ADC for 0.5 V rail-to-rail operation. 19 chips are 1.11 nA and 6.62 mV, respectively. The measured temperature coefficient (TC) in the range of 20 \({}^{\circ}\)C-50 \({}^{\circ}\)C, which is reasonable temperature variation inside the human body, is 0.06%/C and 0.4%/C for voltage and current references, respectively. The measured power supply rejection ratio of the proposed reference generator shows \(<\)\(-\)50 dB at 100 Hz. Figure 20 shows the ISCOM measurement setup using the pre-recorded neural signals. It should be noted that the transmitted signals do not affect or interfere with neural activities because neurons (or neuronal responses) are transparent for high-frequency signals (\(>\)100 kHz). We could obtain a data bandwidth of 10 kb/s data at 160 \(\upmu\)W and measured a channel attenuation of \(-\)17 dB at 10 cm distance from the ISCOM. For feasibility test of the distributed modular system's expandability,  
Figure 19: Measured variations of (**a**) reference current, and (**b**) reference voltage. (**c**) Variation of reference current and voltage as a function of temperature. Figure 18: Measured outputs of the reference generator as a function of power supply voltage variations: (**a**) current reference and (**b**) voltage reference. two neural interface modules were placed by 1 cm apart from each other and the ISCOM\({}_{\text{TX1}}\) and ISCOM\({}_{\text{TX2}}\) signals from two modules were simultaneously transmitted through the skin. As shown in Figure 20b, the pre-recorded signals were transmitted through the skin and retrieved successfully. For this measurement, the current amplitude was set as \(\sim\)10 \(\upmu\)A at the ISCOM output from 1.0 V power supply. The measured bit-error-rate (BER) in this experiment was \(\sim\)10\({}^{-6}\). Figure 21 shows the impedance of the fabricated microelectrode array. The impedance was measured in phosphate-buffered saline (PBS) solution with a Pt counter electrode by an impedance analyzer (Agilent 4192A). Due to the electrical double layer formed in series with the electrode, it shows the capacitive impedance at low frequency. The impedances of the recording and reference electrodes are distributed from \(\sim\)1.2 k\(\Omega\) to 5.6 k\(\Omega\) at 1 kHz and that of the local ground exhibits \(\sim\)60 \(\Omega\) at the same frequency. Since the impedance of the recording and reference electrodes are at least 1000\(\times\) smaller than that of the preamplifier, the variation of impedance does not cause serious distortion in recordings. Table 2 summarizes the performance compared with other ECoG and EEG recording systems. This work shows the state-of-the-art power and noise performance. The NEF and PEF for 10 Hz-10 kHz are the smallest among the reported works due to the increased effective \(G_{m}\) from the double gated push-pull amplifier.