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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 sigma-delta ADC) measures the difference between two inputs.
In a typical single-ended application, the ADC compares the input voltage to a fixed reference voltage (for example, 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; when ADCIN = 5V, the output is expressed as 5V – 2.5V = 2.5V.
Thus, the digital output range corresponds to an ADCIN of 0V to 5V of ±2.5V. The figure below shows: ADC input converter circuit.
The Figure 1 circuit is capable of converting the ±10.5V input signal to the input range (0V to 5V) of the MAX1402 ADC. 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 and R2 resistor dividers convert the input while biasing the input with 3.28V. When the input is grounded, the ADC input is centered at 2.5V (the ADC digital output is 0 when VIN = 0V). The accuracy of the component guarantees the 16-bit accuracy of the ADC.
Single-ended measurements configure the input as two independent channels and compare it to the 2.56V reference of IN6. For higher accuracy, the ADC can be configured as a differential input with one channel as the ground potential detection input.
Configure the MAX1402 as a differential measurement that measures the voltage difference between IN1 and IN2. These inputs accept ±10.5V input voltage, while an internal programmable gain amplifier (PGA) is used to increase small signal resolution. For example, a 4x gain allows the ADC to achieve 16-bit resolution when measuring ±2.625V input signals.
The resistor divider ratio can be varied to accommodate different input ranges, but the same ratio is required to bias the circuit. For example, a 5:1 ratio corresponds to an input range of ±15.0V and a bias of 3.00V. To calibrate the system, simply ground the input and connect the input to a known voltage and record the output value. These two values ​​can be used to calculate the bias and gain coefficients for each input range.