ADC Input Converter Circuit Analysis-EEWORLD

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Many high-precision analog-to-digital converters require an input range between 0.0V and 5.0V. For example, the MAX1402 (18-bit multichannel Σ-Δ ADC) measures the difference between two inputs. In a typical single-ended application, the ADC compares the input voltage to a fixed reference voltage (e.g., 2.500V): when ADCIN = 0V, the digital output represents 0V – 2.5V = -2.5V; when ADCIN = 2.5V, the output represents 2.5V – 2.5V = 0V; and when ADCIN = 5V, the output represents 5V – 2.5V = 2.5V. Thus, the digital output range corresponding to an ADCIN of 0V to 5V is ±2.5V.

The circuit in Figure 1 converts ±10.5V input signals to the input range of the MAX1402 ADC (0V to 5V). The two channels of the ADC (IN1 and IN2 in this case) are configured for fully differential or high-precision single-ended measurements. The R1, R2 resistor divider transforms the input and biases the input with 3.28V. When the input is grounded, the ADC input is centered around 2.5V (when VIN = 0V, the ADC digital output is 0). The precision of the components ensures the 16-bit accuracy of the ADC.

Figure 1. This circuit allows an ADC with an input range of 0V to 5V (single-ended or differential) to process an input range of ±10.5V.

Configuring the MAX1402 for differential measurement allows the voltage difference between IN1 and IN2 to be measured. These inputs accept ±10.5V input voltages, and the internal programmable gain amplifier (PGA) is used to increase the resolution of small signals. For example, a gain of 4 allows the ADC to achieve 16-bit resolution when measuring ±2.625V input signals.

For single-ended measurements, the input can be configured as two independent channels and compared to the 2.50V reference voltage at IN6. For higher accuracy, the ADC can be configured as a differential input with one channel acting as a ground sense input.

The resistor divider ratio can be changed to accommodate different input ranges, but the same ratio needs to be used to bias the circuit. For example, a 5:1 ratio corresponds to a ±15.0V input range and a 3.00V bias. To calibrate the system, simply connect the input to ground, connect the input to a known voltage, and record the output value. These two values can be used to calculate the bias and gain factors for each input range.

Keywords：ADC Reference address：ADC Input Converter Circuit Analysis

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