The conversion of analog signals into digital signals is generally divided into four steps, namely, sampling, holding, quantization and coding. The first two steps are completed in the sample-and-hold circuit and the last two steps are completed in the ADC.

Commonly used ADCs are integration type, successive approximation type, parallel comparison type/serial parallel type, σ -δ modulation type, capacitor array successive comparison type and voltage-frequency conversion type. The following briefly introduces the basic principles and characteristics of several commonly used types:

1 integer type (such as TLC7 135). The working principle of integral ADC is to convert the input voltage into time or frequency, and then get the digital value through timer/counter. Its advantage is that it can obtain high resolution with a simple circuit, but its disadvantage is that the conversion accuracy depends on the integration time, so the conversion rate is extremely low. At first, monolithic ADC mostly used integral type, and now the successive comparison type has gradually become the mainstream. Double integration is a commonly used AD conversion technology, which has the advantages of high precision and strong anti-interference ability. However, the high-precision double-integral AD chip is expensive, which increases the cost of the single-chip microcomputer system.

2 successive approximation type (such as TLC083 1). The successive approximation AD consists of a comparator and an Ada converter through successive comparison logic. Starting from the MSB, the input voltage is sequentially compared with the output of the built-in DA converter for each bit, and the digital value is output after n comparisons. Its circuit scale is medium. Its advantages are high speed, low power consumption and very expensive low resolution (12).

3 parallel comparison type/serial parallel comparison type (such as TLC55 10). Parallel comparison AD uses multiple comparators, and only one comparison is needed to realize the conversion, which is also called FLash type. Because the conversion rate is extremely high, the N-bit conversion requires 2n- 1 comparator, so the circuit scale is particularly large and the price is very high, and it is only suitable for high-speed fields such as video AD converters. Serial-parallel comparison AD is between parallel type and successive comparison type in structure. The most typical one is composed of two n /2-bit parallel AD converters and DA converters, which are converted by two comparisons, so it is called semi-flash type.

4 σ-δ modulation type (such as AD770 1). σ-δADC digitizes analog signals with extremely low sampling resolution (1 bit) and extremely high sampling rate, and improves the effective resolution by using methods such as oversampling, noise shaping and digital filtering, and then samples the ADC output to reduce the effective sampling rate. The circuit structure of σ -δ ADC consists of a very simple analog circuit and a very complex digital signal processing circuit.

5 capacitor array successive comparison type. Capacitance array comparison AD built-in DA converter adopts capacitance matrix method, which can also be called charge redistribution type. In the general resistance array DA converter, the resistance of most resistors must be the same, so it is not easy to produce high-precision resistors on a single chip. If the resistor array is replaced by the capacitor array, a high-precision monolithic AD converter can be manufactured at low cost. The recent continuous comparison AD converters are mostly capacitor arrays.

6 voltage-frequency conversion type (such as AD650). Voltage-frequency conversion type realizes analog-to-digital conversion through indirect conversion. Its principle is to first convert the input analog signal into frequency, and then use the counter to convert the frequency into digital quantity. Theoretically, the resolution of this kind of AD can be improved almost infinitely, as long as the sampling time can meet the width of the cumulative pulse number required by the output frequency resolution. Its advantages are high resolution, low power consumption and low price, but external counting circuit is needed to complete AD conversion.

Basic principle of digital-to-analog converter (DAC)

There is not much difference in the internal circuit composition of DAC, which is generally classified according to whether the output is current or voltage and whether it can be multiplied. Most DACs consist of a resistor array and n current switches (or voltage switches). The switch is switched according to the digital input value to generate a current (or voltage) proportional to the input. In addition, in order to improve the accuracy, a constant current source is added to the device. Dacs can be divided into two categories: voltage-mode dacs and current-mode dacs. Voltage source DAC includes resistance network, T-type resistance network and tree switch network. Current-mode DAC can be divided into current-mode resistor network and inverted T-type resistor network.