Many A/D converters operate exclusively in a 5V single-supply environment. Typically, the input range of these converters is between the power supply voltages, ground and 5V. Additionally, they only accept single-sided input signals or differential signals with some limitations. The voltage range of the inverting input of these pseudo-differential converters is several hundred millivolts. Because of their lower conversion rates, of approximately 1MHz at best, these devices usually service markets such as process control, data acquisition or battery operated systems.
An alternative to these single-ended input devices is a true-differential input device, such as the single supply, MCP3301, 12-bit plus sign, SAR A/D converter from Microchip Technology. This device still services the same markets, but the true differential input of this A/D converter opens the door for a large array of single supply application solutions. With the MCP3301 the ranges of both inputs of the converter are from rail to rail. The inputs of the A/D converter are simultaneous sampled and the conversion of the differential input signal to the converter enhances the accuracy of the device by one bit.
One configuration that takes advantage of this type of input stage is shown in Figure 1. A resistive network in front of this differential A-D converter, in conjunction with the reference voltage and a few resistors, implements differential input ranges that operate beyond the power supply voltages of the converter. This design technique works only when the A/D converter has a full-differential input. Dependant on the ratio between R1X and R2X, this circuit is useful when measuring a variety of voltages below ground and above the power supply levels even though the power supply voltages range is between ground and 2.7 Volts to 5V.
Figure 1: The resistive network and voltage reference to the MCP3301 12-bit plus sign ADC converter increases the input range beyond the power supply range.
In Figure 1, the input range of the two inputs (IN(+) and IN(-)) of the MCP3301 is from ground to the power supply voltage. The input voltage values of either input can be anywhere between the power supply rails. When a conversion is complete, the binary code for the difference between these two pins has a binary twos-complement format. In order to comply with the input range requirements of the MCP3301, the voltage divider attenuates (R1X and R2X) the analog signal from VIN+ and VIN- to V'IN+ and V'IN- in relation to the reference voltage. These dividers and the voltage reference keep the signals within the minimum and maximum voltage ranges of the MCP3301 as well as keep the differential input range less than or equal to VREF. The following formula determines the ratio of the resistors, R1X and R2X, where the input ranges are equally balanced with respect to ground:
The combination of the resistors and voltage reference should not force the absolute voltage at the inputs of the A/D converter beyond the positive and negative power-supply rails. Once the resistors are chosen the input voltages of the converter can be checked with these formulas:
Now that the relationship between R1X and R2X is set, the actual values of these resistors are dependent on the A-D conversion speed that will be used in the application. The range of these resistors should be low enough so the source or external input resistance does not interfere with the A/D converter accuracy. The input (external) resistance in this application is:
The parallel combination of larger value resistors increases input (external) resistance. A higher value of input (external) resistance can affect the gain accuracy of conversions given a clock frequency rate. The relationship between the input resistance and the clock frequency for the MCP3301 is shown in Figure 2. An input resistance greater than ~7k-ohms and/or the clock frequency more than 1MHz compromises the gain accuracy of the MCP3301 A/D converter.
Figure 2: In order to preserve 12-bit plus sign accuracy, the input (external) resistance versus conversion clock frequency should be adjusted on the MCP3301 SAR converter accordingly. This graph indicates what the maximum conversion frequency can be versus the input (external) resistance that is needed in order to keep errors to less than 1LSb.
The gain accuracy of the circuit is dependent on the accuracy of the external resistors, R1X and R2X and the A/D converter. A typical tolerance value for low cost resistors is 1 percent. This specification is well below the gain specifications of the A/D converter and is easily calibrated out with the microcontroller. Other A/D specifications such as offset error, differential linearity and integral linearity are not effected by this input resistive voltage divider.
Table 1 shows some typical 1 percent resistance values for a +/-10 Volt input range when the MCP3301, 12-bit A/D converter is used in the circuit of Figure 1. The circuit in this example uses a 5V-power supply. The table also lists the widest voltage ranges seen at the ADC input stage for various voltage references.
Table 1: Typical resistor combinations, +/–10 Volt input range.
Finally, this test circuit uses a PIC16F73, 8-bit CMOS Flash microcontroller. Data instructions are written to a LCD array for viewing. The PIC16F73 microcontroller can operate with a 20MHz clock rate, giving an instruction cycle of 200ns. This device also has a Synchronous Serial Port that supports SPITM and I2CTM. The SPI port was used to communicate with the MCP3301 A/D converter.
This configuration easily expands the application possibilities of a classical, somewhat restricted input range of a single supply SAR A/D converter, to include a broader range of possibilities. In addition, other circuits can be implemented with this 12-bit plus sign, true differential A/D converter. Circuits such as level shifting, converting differential input signals and single pole signals and zero low pass and high pass filters.
“Driving the Analog Inputs of a SAR A/D Converter,” Baker, Bonnie C., AN246, Microchip Technology, Inc.