Converter refers to a device that converts one signal into another. Signal is the form or carrier of information. In automatic instrumentation equipment and automatic control systems, one signal is often converted into another signal compared with the standard or reference value in order to connect the two types of instruments. Therefore, the converter is often two instruments (or Device).


AD converter introduction

  1. Classification of AD converters
    The following briefly introduces the basic principles and characteristics of several commonly used types: integral type, successive approximation type, parallel comparison type/serial parallel type, sigma-delta modulation type, capacitor array successive comparison type and voltage-frequency conversion type.
    1) Integral type (such as TLC7135)
    The working principle of integral AD is to convert the input voltage into time (pulse width signal) or frequency (pulse frequency), and then the digital value is obtained by the timer/counter. The advantage is that high resolution can be obtained with a simple circuit, but the disadvantage is that because the conversion accuracy depends on the integration time, the conversion rate is extremely low. Most of the initial single-chip AD converters used integral type, and now the successive comparison type has gradually become the mainstream.
    2) Successive comparison type (such as TLC0831)
    The successive comparison AD is composed of a comparator and a DA 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 a digital value is output after n comparisons. The circuit scale is medium. Its advantages are high speed, low power consumption, and high price at low resolution (12 bits).
    3) Parallel comparison type/serial parallel comparison type (such as TLC5510)
    Parallel comparison type AD uses multiple comparators to perform conversion only for one comparison, also known as FLash (fast) type. Due to the extremely high conversion rate, n-bit conversion requires 2n-1 comparators. Therefore, the circuit scale is also very large and the price is high. It is only suitable for areas with extremely high speeds such as video AD converters.
    The structure of the serial-parallel comparison type AD is between the parallel type and the successive comparison type. The most typical one is composed of two n/2-bit parallel type AD converters and DA converters. The conversion is carried out by two comparisons, so it is called It is a Half flash (semi-fast) type. There are also three or more steps to achieve AD conversion called multistep (Multistep/Subrangling) type AD, and from the perspective of conversion timing can also be called pipeline (Pipelined) type AD, modern hierarchical AD also added multiple The conversion result is used for digital operation to modify features and other functions. This type of AD speed is higher than the successive comparison type, and the circuit scale is smaller than the parallel type.
    4) Σ-Δ (Sigma?/FONT>delta) modulation type (such as AD7705)
    The Σ-Δ AD is composed of an integrator, a comparator, a 1-bit DA converter, and a digital filter. In principle, it is similar to the integral type. The input voltage is converted into a time (pulse width) signal and processed by a digital filter to obtain a digital value. The digital part of the circuit is basically easy to single-chip, so it is easy to achieve high resolution. Mainly used for audio and measurement.
    5) Capacitor array successive comparison type
    The capacitor array successive comparison type AD adopts the capacitor matrix method in the built-in DA converter, which can also be called the charge redistribution type. The values ​​of most resistors in general Resistor array DA converters must be consistent, and it is not easy to generate high-precision resistors on a single chip. If a capacitor array is used instead of a Resistor array, a high-precision monolithic AD converter can be made at low cost. Most of the recent successive comparison AD converters are of the capacitor array type.
    6) Voltage-frequency conversion type (such as AD650)
    Voltage-Frequency Converter (Voltage-Frequency Converter) implements analog-to-digital conversion through indirect conversion. The principle is to first convert the input analog signal into a frequency, and then use a counter to convert the frequency into a digital quantity. In theory, the resolution of this AD can be increased 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 it requires an external counting circuit to complete the AD conversion together.
  2. The main technical indicators of the AD converter
    1) Resolution (Resolution) refers to the amount of change in the analog signal when the digital value changes by a minimum amount, defined as the ratio of full scale to 2n. Resolution is also called accuracy, usually expressed by the number of digits of the digital signal.
    2) Conversion Rate (Conversion Rate) refers to the reciprocal of the time required to complete an AD conversion from analog to digital. The conversion time of integral AD is milliseconds, which is low-speed AD, successive comparison AD is microseconds and medium-speed AD, and full-parallel/serial-parallel AD can reach nanoseconds. Sampling time is another concept, which refers to the interval between two conversions. In order to ensure the correct completion of the conversion, the sample rate (Sample Rate) must be less than or equal to the conversion rate. Therefore, it is acceptable for some people to equate the conversion rate numerically with the sampling rate. Commonly used units are ksps and Msps, which means kilo / Million Samples per Second (kilo / Million Samples per Second).
    3) Quantizing Error (Quantizing Error) The error caused by the finite resolution of AD, that is, the maximum between the stepped transfer characteristic curve of finite resolution AD and the transfer characteristic curve (straight line) of infinite resolution AD (ideal AD) deviation. It is usually 1 or half of the smallest digital analog variation, expressed as 1LSB, 1/2LSB.
    4) Offset Error The value at which the output signal is not zero when the input signal is zero, can be adjusted to the minimum by an external potentiometer.
    5) Full Scale Error (Full Scale Error) The difference between the corresponding input signal and the ideal input signal value at full-scale output.
    6) Linearity (Linearity) The maximum deviation between the transfer function of the actual converter and the ideal straight line, excluding the above three errors.
    Other indicators include: Absolute Accuracy, Relative Accuracy, Differential Nonlinearity, Monotonicity and Error-Free Code, Total Harmonic Distotortion (THD) and integral nonlinearity.
  3. DA converter
    There is not much difference in the internal circuit composition of the DA converter, and it is generally classified according to whether the output is current or voltage, and whether it can be multiplied. Most DA converters consist of a resistor array and n current switches (or voltage switches). Switch the switch according to the digital input value to generate a current (or voltage) proportional to the input. In addition, there are also constant current sources placed inside the device in order to improve accuracy. Generally speaking, because the switching error of the current switch is small, most of the current switch type circuits are used. If the current switch type circuit directly outputs the generated current, it is a current output type DA converter. In addition, the voltage switch type circuit is a direct output voltage type DA converter.
    1) Voltage output type (such as TLC5620)
    Although there are voltage output DA converters that directly output voltage from a resistor array, they generally use a built-in output amplifier to output with low impedance. Devices that directly output voltage are only used for high-impedance loads. Because there is no delay in the output amplifier part, they are often used as high-speed DA converters.
    2) Current output type (such as THS5661A)
    Current output type DA converters seldom use current output directly, and most of them are connected to a current-voltage conversion circuit to obtain voltage output. The latter has two methods: one is to only connect a load resistor to the output pin to perform current-voltage conversion; It is an external operational amplifier. The method of current-voltage conversion with load resistance, although voltage can appear on the current output pin, it must be used within the specified output voltage range, and due to the high output impedance, it is generally used with an external operational amplifier. In addition, most CMOSDA converters cannot operate correctly when the output voltage is not zero, so an external operational amplifier must be connected. When an external operational amplifier is used for current-to-voltage conversion, the circuit configuration is basically the same as the voltage output type of the built-in amplifier. At this time, due to the addition of the delay of the DA converter to the current set-up time of the DA converter, the response becomes slower . In addition, the operational amplifier in this circuit is prone to oscillate due to the internal capacitance of the output pin, and phase compensation is sometimes necessary.
    3) Multiplication type (such as AD7533)
    Some DA converters use a constant reference voltage, and some add an AC signal to the reference voltage input. The latter is called a multiplying DA converter because it can get the result of multiplying the digital input and the reference voltage input. The multiplication type DA converter can generally not only perform multiplication, but also can be used as an attenuator that digitally attenuates the input signal and a modulator that modulates the input signal. [1]
    4) A DA converter
    The one-bit DA converter is completely different from the aforementioned conversion method. It converts the digital value into a pulse width modulation or frequency modulation output, and then uses a digital filter for averaging to obtain a general voltage output (also known as a bit stream method). For audio and other occasions.
  4. The main technical indicators of DA converter:
    1) Resolution (Resolution) refers to the ratio of the minimum analog output (corresponding to digital quantities only the lowest bit is ‘1’) to the maximum (corresponding digital quantities are all valid digits ‘1’)
    2) Setting Time is the time required to convert a digital quantity into a stable analog signal, and it can also be considered as the conversion time. The settling time is often used in DA to describe its speed, rather than the conversion rate commonly used in AD. Generally, the settling time of the current output DA is shorter, and the voltage output DA is longer.