# Minimally-Invasive Neural Interface for Distributed Wireless Electrocorticogram Recording Systems
## 5 Measurement Results
### System Characteristics Measurement  
The preamplifier dissipates 0.5 uW at 1 V power supply while occupying 500 \(\times\) 180 u\(\mathrm{m}^{2}\) of the area. As shown in Figure 14, the measured mid-band gain is \(\sim\)37.5 dB. The low and high corner \begin{table}
\begin{tabular}{l l} \hline \hline
**Parameters** & **Measured Results** \\ \hline \multicolumn{2}{l}{**Recording Integrated Circuit**} \\ \hline Supply voltage (internally generated) & 0.5/1.0 V \\ Supply voltage (externally supplied) & 1.5–3.3 V (Li-Ion rechargeable battery) \\ Power/Ch. & 2.5 uW \\ Gain & 100–1000 V/V (variable) \\ NEF/PEF & (1–500 Hz) & 5.2/27.04 \\  & (10 Hz to 10 kHz) & 1.69/2.86 \\ Resolution & 8 bit \\ CMRR & 35 dB \\ PSR & 48 dB \\ Active area & 3.2 mm \(\times\) 0.9 mm (with ISCOM) \\ \hline \multicolumn{2}{l}{**Wireless Communication**} \\ \hline Communication method & Intra-Skin Communication (ISCOM) \\ Supply voltage & 1.0 V \\ Modulation & FSK \\ Data bandwidth & \(>\)100 kb/s \\ Link gain & –17 dB \\ Power & 160 uW \\ Carrier frequency & 2–4 MHz \\ Bit error rate & \(10^{-6}\) \\ \hline \multicolumn{2}{l}{**Flexible Microelectrode Array**} \\ \hline Materials & Pt on Parylene-C \\ Number of recording/reference channel & 16/4 \\ Impedance of electrodes @ 1 kHz & 1.2–5.6 k\(\Omega\) \\ Electrode size & 400 u\(\mathrm{m}\) in diameter \\ Total Area & 18 mm \(\times\) 18 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of characteristics of the fabricated IC and microelectrode array. Figure 13: Microphotograph of the fabricated neural interface circuit chip. frequencies can be digitally programmed from 0.5 Hz to 2.2 kHz and 1.5 kHz to 31 kHz, respectively, for various applications. The measured input-referred noises (IRN) are 5.62 \(\upmu\)Vrms for 10 Hz to 10 kHz and 4.26 \(\upmu\)Vrms for 1 Hz to 0.5 kHz, respectively. From the measured power consumption and IRNs, NEFs of 1.69 and 5.2 are calculated for two different frequency bands. To verify the IRN reduction by the implemented quasi-floating body gate scheme, the IRNs for both with and without the quasi-floating body gate connections are measured and compared in Figure 15. It clearly shows the thermal noise floor is reduced from 52 nV/\(\upmu\)Hz to 47 nV/\(\upmu\)Hz, approximately 10% reduction. Therefore, we can estimate that the proposed preamplifier architecture can effectively save \(\sim\)20% of power reduction at the same noise performance. The ADC core occupies 228 \(\times\) 180 \(\upmu\)m\({}^{2}\) with a unit capacitance of 49 fF. As shown in Figure 16a, the measured INL and DNL are 0.70/\(-\)0.75 LSB and 0.3/\(-\)0.5 LSB, respectively. The measured FFT spectrums normalized to the maximum allowable input signals of 1.9474 kHz and 12.7621 kHz at 31.25 kS/s are also shown in Figure 16b. The signal-to-noise and distortion ratio (SNDR) is 45.14 dB for the Nyquist input signal at 31.25 kS/s. Figure 17a shows the SNDR and spurious-free dynamic range (SFDR) for various sampling frequencies while maintaining the input frequencies as the half of the sampling frequency. In the low-frequency regime, especially in the ECoG band (<1 kHz), the SFDR is over 70 dB, which is suitable for ECoG recordings [36]. The measured power consumption of each individual block in the SAR ADC is also plotted in Figure 17b. The ADC consumes 87.41 nW at the maximum sampling rate of 31.25 kS/s at 0.5 V supply. The fabricated ADC shows an effective number of bit (ENOB) of 7.21 and 20 fJ/c-s in the figure of merit (FoM) for rail-to-rail operation. Figure 14: Measured frequency response of the LNA: (**a**) low frequency corner sweep and (**b**) high frequency corner sweep. Figure 15: Measured power spectral densities of the input referred noise of the LNA with/without body input. The fabricated reference generator operates from 1.2 to 3.3 V power supplies while consuming 120 nW at 1.5 V and occupying 0.011 mm\({}^{2}\) in area. From the rechargeable battery output specification of 1.6 to 3.0 V, the reference generator can reliably produce the required bias.