# A Single-Bit Incremental Second-Order Delta-Sigma Modulator with Coarse-Fine Input Buffer
## 2 Proposed Delta Sigma Modulator with Coarse-Fine Input Buffer
### Coarse-Fine Input Buffer  
A coarse-fine input buffer was used for fast, low-noise, characteristic-based analog signal acquisition. As shown in Figure 4, the input buffer comprises a parallel connection between the coarse and fine buffers and a timing circuit. The main characteristics of coarse circuits are that they are faster and noisier than fine buffers. To compensate for this, a low-noise but slow fine buffer was connected in parallel. The acquisition of analog signals with high speed and low noise can be achieved through two complementary and timing circuits. The circuit had two inputs and two outputs; the output was fed to a DSM for the post-processing of the acquired analog signal. For low-noise characteristics, a fine buffer must have high voltage gain characteristics.  
The chopper technique was used to reduce flicker noise in the low-frequency band [22; 23] in the fine buffer. The input chopper modulates the input signal to a higher frequency and prevents degradation due to the low-frequency noise. The output chopper demodulates the input signal to the baseband and also modulates the low-frequency noise of the amplifier to the high-frequency band; thus, a high signal-to-noise ratio can be achieved.  
The coarse buffer was designed to operate with the unit-gain bandwidth of 3 MHz. The bandwidth of the fine buffer is limited by the chopper frequency. The chopper is operated at 16 kHz. The output stage of the fine and coarse buffers is designed using the Monticelli-style class-AB amplifier circuit [24] to enhance the driving capability of the switching load.  
The amplifier used for the coarse buffer is shown in Figure 5. The circuit was a single-ended Monticelli amplifier circuit. A rail-to-rail input output-type circuit was used for a wide range of analog signal inputs and outputs. A Miller compensation capacitor is used from the class-AB output terminal for the stable operation of the circuit.  
Figure 4: Schematic of coarse-fine input buffer.  
The amplifier used in the fine buffer is shown in Figure 6. The circuit structure is similar to that of the coarse buffer circuit. However, a chopper is used between the input and output stages to focus on the low-noise characteristics. In the input stage, the signal was modulated using a chopper, and the folded cascode stage was demodulated. There is a difference in the total amount of current compared to the coarse buffer. Low-noise characteristics can be achieved by adjusting the input differential amplifier stage and the folded-cascode stage current ratio. To optimize the \(\mathrm{g_{m}}\) value, which is important for optimizing low-noise characteristics, a fine buffer can achieve low-noise characteristics. Coarse and fine buffers have similar circuit structures but complementary characteristics. The coarse buffer is responsible for operating characteristics over a wide frequency range, while the fine buffer has high voltage gain and low noise characteristics. Figure 7 shows the transient simulation results for the optimal timing of each buffer. The enable time of the coarse buffer is slightly shorter than that of the fine buffer. Two buffer enable times of approximately 1 \(\upmu\)s were shared. Both enable the signals to operate at a frequency of 125 kHz. The signal is stabilized by the fine buffer after the pre-charge of the coarse buffer with a fast settling time.  
Figure 5: Schematic of the coarse input buffer.  
Figure 6: Schematic of the fine input buffer.