# Direct type ADC

Linear integrated circuit applications 2020-11-20 01:16:08# Direct-type ADC

An analog-to-digital converter ** (ADC) ** converts an analog signal to a digital signal. The digital signal is represented by a binary code, which is a combination of bits 0 and 1.

The ** block diagram ** of an ADC is shown in the following figure -

Observe that in the figure above, an analog-to-digital converter ** (ADC) ** consists of a single analog input and many binary outputs. Usually the number of binary outputs of the ADC will be a power of two.

There are ** two types ** of ADCs: Direct type ADCs and Indirect type ADCs. This chapter deals in detail with type ADCs. Direct.

If the ADC performs the analog-to-digital conversion directly using the equivalent internally generated digital (binary) code for comparison with the analog input, then it is called as ** Direct-type ADC **.

Here are ** examples ** of direct-type CAN -

This section describes these Direct type ADCs in detail.

## ADC counter type

An ** ADC counter type ** produces a digital output, which is approximately equal to the analog input using the internal counter operation.

The ** block diagram ** of an ADC type counter is shown in the following figure -

The ADC counter type mainly consists of 5 blocks: clock signal generator, counter, DAC, Comparator and Control Logic.

The ** operation ** of a counter type ADC is as follows -

- The
**control logic**resets counter and activates the clock signal generator to send the clock pulses to the uh counter, when it has received the start command signal. - The
**counter**is incremented by one for each clock pulse and its valueeur will be in binary (digital) format. This counter output is applied as an input to the DAC. -
**DAC**converts the received binary (digital) input, which is the output of the counter, into an analog output. The comparator compares this analog value, $ V_ {a} $ with the external analog input value $ V_ {i} $. - The
**comparator output**will be**'1 '**as long as 𝑉𝑖 is greater than. The operations mentioned in the two steps above will be continued as long as the control logic receives "1" from the comparator output. - The
**comparator output**will be**'0 '**when $ V_ {i} $ is less than or equal to $ V_ {a} $. Thus, the control logic receives "0" from the output of the comparator. Then the control logic turns off the clock signal generator so that it does not send any clock pulses to the counter. - At this moment, the output of the counterwill be displayed as
**digital output**. It is almost equivalent to the corresponding external analog input value $ V_ {i} $.

## Successive approximation ADC

A ** type of successive approximation ADC ** produces digital output , which is approximately equal to the analog input using the successive approximation technique internally.

The ** block diagram ** of a successive approximation ADC is shown in the following figure

Successive approximation ADC mainly consists of 5 blocks - Clock signal generator, successive approximation register (SAR ), DAC, comparator and control logic.

The ** works ** of a successive approximation ADC is as follows -

- The
**control logic**resets all bits of the SAR and activates the generator clock signal in in order to send the clock pulses to the SAR , when it has received the start command signal. - Binary (digital) data present in
**SAR**will be updated for each clock pulse based on the comparator output. The output of the SAR is applied as an input of the DAC. -
**The DAC**converts the received digital input, which is the output of the SAR, into an analog output signal. The comparator compares this analog value $ V_ {a} $ with the value of external analog input $ V_ {i} $. - The
**output of a comparator**will be '1 ' as long as $ V_ {i} $ is greater than $ V_ {a} $. Likewise, the comparator output will be '0 ', when $ V_ {i} $ is less than or equal to $ V_ {a} $. - The operations mentioned in the above steps will be continued until the digital output is valid.

The digital output will be valid, when it is almost equivalent to the corresponding external analog input value $ V_ {i} $.

## Flash-type ADC

A flash type ** ADC ** produces an equivalent digital output for a corresponding analog input in no time. Therefore, flash-type ADC is the fastest ADC.

The ** circuit diagram ** of a 3-bit flash type ADC is shown in the following figure -

The 3-bit ADC flash consists of a network

The ** operation ** of a 3-bit flash type ADC is as follows.

- The
**network of**contains 8 equal resistors. A reference voltage $ V_ {R} $ is applied to this whole network by The voltage drop across each resistor from bottom to top with respect to ground will be integer multiples (from 1 to 8) of $ frac {V_ {R}} {8} $. - The
**external input voltage**$ V_ {i} $ is applied to the non-inverting terminal of all comparators. The voltage drop across each resistor for m lowup with respect to ground is applied to the inverting terminal of the comparators from the bottom up. - At the same time, all comparators compare the external input voltage with the voltage drops present on the other respective input terminal. This means that the comparison operations take place by each comparator
**parallelly**. - The
**comparator output**will be '1 As long as $ V_ {i} $ is greater than the voltage drop present on the other respective input terminal. Likewise, the output of the comparator will be '0 ', when $ V_ {i} $ is less than or equal to the voltage drop present on the other respective input terminal. - All comparator outputs are connected like
**priority encoder**inputs. This priority encoder produces a binary code (digital output), which corresponds to the high priority input which has '1 '. - Therefore, the priority encoder output is notis nothing other than the binary equivalent
**(digital output)**of the external analog input voltage, $ V_ {i} $.

Flash type ADC is used in applications where the conversion speed from analog input to digital data needs to be very high.