Four types of analog-to-digital converter
Original article date: September 1997
Many different types of analog-to-digital converters are available. Differing ADC types offer varying resolution, accuracy and speed specifications. This assistance from IOTech’s new definitive guide.
All ADCs convert an analog voltage to a digital number. The most popular ADC types are:
- the parallel (flash) converter
- the successive approximation ADC
- the voltage-to-frequency ADC
- the integrating ADC.
The digital number represents the input voltage in discrete steps with finite resolution. ADC resolution is determined by the number of bits that represent the digital number. A n-bit ADC has a resolution of 1 part in 2n. For example, a 12-bit ADC has a resolution of 1 part on 4096 (212= 4096). Twelve-bit ADC resolution corresponds to 2.44mV for a 10V range. Similarly, a 16-bit ADC’s resolution is 1 part in 66 536 (216= 65 536), which corresponds to 0.153mV for a 10V range.
The parallel converter is the simplest ADC implementation. It uses a reference voltage at the full-scale of the input range and a voltage divider. The latter is composed of 2n+ 1 resistors in series, where n is the ADC resolution in bits. The value of the input voltage is determined by using a comparator at each of the 2nreferences voltages created in the voltage divider.
Flash converters are very fast (up to 500MHz) because the bits are determined in parallel. This method requires a large number of comparators, thereby limiting the resolution of most parallel converters to 8 bits (256 comparators). Flash converters are commonly found in transient digitisers and digital oscilloscopes.
A successive approximation ADC employs a digital-to-analog converter (DAC) and a single comparator. It effectively makes a bisection or binomial search by beginning with an output of zero. It provisionally sets each bit of the DAC, beginning with the most significant bit. The search compares the output of the DAC to the voltage being measured. If setting a bit to one causes the DAC output to rise above the input voltage, that bit is set to zero.
Successive approximation is slower than flash conversion, because the comparisons must be performed in a series and the ADC must pause at each step to set the DAC and wait for it to settle. However, conversion rates over 200kHz are common. Successive approximation is relatively inexpensive to implement for 12 and 16-bit resolution. Consequently, they are the most commonly used ADCs and can be found in many PC-based data acquisition products.
Voltage-to-frequency ADCs convert an input voltage to an output pulse train with a frequency proportional to the input voltage. Output frequency is determined by counting pulses over a fixed time interval and the voltage is inferred from the known relationship.
Voltage-to-frequency conversion has a high degree of noise rejection, because the input signal is effectively integrated over the counting interval. Voltage-to-frequency conversion is commonly used to convert slow and often noisy signals.
It is also useful for remote sensing applications in noisy environments. The input voltage is converted to a frequency at the remote location and the digital pulse train is transmitted over a pair of wires to the counter. This eliminates the noise that can be introduced in the transmission of an analog signal over a long distance.
A number of ADCs use integrating techniques, which measure the time to charge or discharge a capacitor to determine input voltage. Dual-slope integration is a common integration technique: using a current that is proportional to the input voltage, a capacitor is charged for a fixed time period. The average input voltage is determined by measuring the time required to discharge the capacitor using a constant current.
Integrating the ADC input over an interval reduces the effect of noise pickup to the AC line frequency if the integration time is matched to a multiple of the AC period. For this reason, it is commonly used in precision digital multimeters and panel meters. Twenty-bit accuracy is not uncommon. This disadvantage is a relatively slow conversion rate (60Hz maximum, slower for ADCs that integrate over multiple line cycles).
| ADC Type | Typical Resolution | Typical Speed |
| Parallel Converter | 4-8 bit | 100kHz to 500MHz |
| Successive Approximation | 8-16 bit | 10kHz to 1MHz |
| Voltage-to-Frequency | 8-12 bit | 1-60Hz(*) |
| Integratin g | 12-24 bit | 1-60Hz(*) |
(*)With line cycle rejection
- Any industry that performs monitoring or testing has an array of transducers designed for its measurement requirements. Scensys has released Iotech’s 128-page publication The Signal Conditioning & PC-based Data Acquisition Handbook, which provides practical information for dealing with the most frequently encountered transducers and their associated signals.
- Scensys
- Tel: 01296 397676
- Fax: 01296 397878
- Contact: Earle Donaghy
September 1997