## 3 Measurement Results  
The switched 4-to-1 TCA was implemented in a 0.18 \(\upmu\)m CMOS technology. The core occupies an area of 92 \(\upmu\)m \(\times\) 68 \(\upmu\)m, and the die size including I/O pads is 1000 \(\upmu\)m \(\times\) 840 \(\upmu\)m. A microphotograph of the fabricated chip with bond wires on the test fixture is shown in Figure 10. All the biases were applied through bond wires, and short pulse response measurement was performed with a coaxial micro-receptacle (CMJ) connector.  
The fabricated chip was mounted on a wire-bonded chip-on-board (COB) module to measure the electrical pulse response, as shown in Figure 11a. A 10-k\(\Omega\) resistor acts as a voltage-to-current converter. To measure the pulse transient response of the fabricated circuit, an electrical pulse signal,  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Stage** & **Gain** & **Bandwidth** & **Direct Current (DC)** \\ \hline Current buffers and first combining stages (\(R_{1}\)) & 70 dB\(\Omega\) (3 k\(\Omega\)) & 330 MHz & 1.48 mA \\ Second combining stage & 2 dB & 310 MHz & 0.65 mA \\ Post-amplifier & 6 dB & 415 MHz & 2.06 mA \\ Balun and output buffer & \(-\)10 dB & 353 MHz & 3.78 mA \\ \hline \hline \end{tabular}
\end{table}
Table 1: Summary of the circuit designed performances.  
Figure 10: Microphotograph of the fabricated chip with bond wires on the test fixture.  
Figure 9: Simulated transimpedance gain of the proposed TCA obtained using this balun and output buffer.  
generated from an Agilent 81110A pattern generator (Keysight, Santa Clara, CA, USA), was applied to each input channel of the implemented test fixture, as shown in Figure 11b. The OUT+ and OUT-signals were measured using an Agilent DSO7104B oscilloscope.  
Figure 12 shows the measured transient pulse response for the fabricated chip. The pulse magnitude of the input voltage from the pattern generator is adjusted so that the input current becomes 20 uA and the FWHM of the input signal is 2.2 ns with a 1.8 ns rise time, which is the minimum rise time of the Agilent 81110A pattern generator. The input current is applied to each successive input channel from IN1 through IN4, and 200 segment pulse responses of OUT+ and OUT- are measured. The blue lines represent the pulse response of 200 segments, and the red line represents the average pulse response value. The average peak-voltage of the measured output pulses is approximately 47.4 mV, and hence, the transimpedance gain is calculated to be 67.5 dB\(\Omega\) using by (4). This value is very near the simulated results of 68 dB\(\Omega\).  
\[Z_{T}=20\times\log_{10}\left(\frac{47.4\text{ mV}}{20\text{ uA}}\right) \tag{4}\]  
Figure 11: (a) TCA-chip on board (COB) schematic for electrical pulse response; (b) measurement setup for electrical pulse response.  
Figure 12: Measured transient pulse response for the fabricated chip.  
The integrated single-ended output noise of the switched 4-to-1 TCA was measured via the oscilloscope root-mean-square (RMS) calculation function with no input signal source, as shown in Figure 13[21]. The standard deviation of the output was measured to be 0.524 mV. After subtracting the inherent oscilloscope noise of 0.4 mV\({}_{\text{rms}}\), the corrected single-ended integrated output noise of the TCA was estimated to be 0.338 mV\({}_{\text{rms}}\). The integrated input-referred noise of the differential output of the switched 4-to-1 TCA for each input channel can be calculated