# Design and Signal-Decoding Test Verification of Dual-Channel Round Inductsyn Decoding Circuit
## 3 Decoding-Circuit Hardware Design
### Design of Induced-Signal Processing Circuit  
The induced signal is extracted from the wiring terminals on the stator of a round inductsyn, with two channels, namely coarse and fine. Each channel has four terminal outputs for the induced signal, providing differential signals for sine and cosine. Therefore, for the processing circuit of the induced signal, it is necessary to design eight sets of circuits that meet the design specifications to process the output signals into signals that meet the input requirements of AD2S1210. For the induced-signal processing circuit, a design process is adopted that involves amplification, filtering, and phase adjustment to maximize the preservation of the original characteristics of the induced signal. The processing circuits for the eight sets of differential signals in the coarse and fine channels are essentially the same. However, due to the different output amplitudes of the coarse- and fine-channel-induced signals, as well as minor errors between components, slight adjustments in the values of some resistors and capacitors may be necessary. Additionally, adjustable resistors are used for some components, eliminating the need to modify the hardware circuit when individually debugging each signal in the future. Figure 5 shows the schematic diagram of the induced-signal processing circuit. It only presents the processing circuit for one channel of the coarse-channel \(SIN+\) induced signal, with the other signal processing circuits being essentially the same as this one. The first stage of the induced-signal processing circuit is an amplification circuit. In this paper, we use the high-precision instrumentation amplifier AD620 produced by ADI to build the circuit. The AD620 features low DC offset, low drift, low noise, high common-mode rejection ratio, and high input impedance. It has a gain range of 1-10,000, and the external  
Figure 5: Schematic diagram of the induced-signal processing circuit. circuit configuration is very simple. Gain adjustment can be achieved by changing external resistors. In Equation (3), \(G\) represents gain, and \(R_{g}\) represents the adjusting resistor. \[G=\frac{49.4k\Omega}{R_{g}}+1 \tag{3}\]  
To amplify the differential signal of the coarse channel, which is 20 mVp-p, to meet the specified range of 3.2 Vp-p \(\pm\) 0.5 V, a gain setting between 140 and 180 is required. However, considering the impact of subsequent filtering circuits and phase adjustment circuits on the overall gain, some adjustment margin should be left for the amplification circuit. Therefore, this paper adopts a design of a fixed resistor in series with a potentiometer as the gain adjustment resistor. This is shown in the connection between pins 1 and 8 of U1 in the schematic diagram of the induced signal processing circuit, as shown in Figure 5. For the coarse-channel \(SIN-\) and \(COS+/COS-\), the peripheral configuration circuit of AD620 is the same. However, since the original induced-signal output amplitude of the fine channel is approximately half that of the coarse channel, it is necessary to appropriately modify the fixed resistor and potentiometer pairing in the gain adjustment section to increase the gain. Due to the electromagnetic induction process between the stator and rotor of the round inductsyn, noise signals will inevitably be generated. Although the amplification circuit can filter out noise to a certain extent, noise will also be introduced during the signal transmission process. Excessive noise can cause more spikes in sine signals, and in severe cases, it can alter the sine characteristics of the signal. Therefore, the second stage of the induced-signal processing circuit is a filtering circuit. The filtering circuit is built around the AD8694 operational amplifier produced by Analog Devices (ADI). Two operational amplifiers from the AD8694 chip are used to configure the peripheral circuit.