Counter Type ADC - Ramp Analog to Digital Converter Working , Circuit

The counter type ADC analog to digital converter is a simple ADC also known as a staircase or ramp analog to digital converter. It, as its name suggests, uses a binary counter for the conversion of analog signals. The schematic consists of a comparator, counter, digital to analog converter, a control circuit, AND gate and output latches.

The article is an introduction to the Counter type analog to digital converter working, advantages and disadvantages.

Why is Counter ADC also called Ramp ADC?

The counter type ADC is also termed as digital Ramp analog to digital converter because of the fact that the output of the counter is passed to the digital to analog converter. Whenever the binary counter increments its count, the digital to analog converter increases in the form of a ramp where the ramp looks like a staircase.

Main Components of Counter Type ADC

Followings are the main components of ramp type analog to digital converter circuit:

N-bit Digital Counter
Comparator Circuit
AND Gate
Latches
DAC

Counter type ADC Working

This section will provide an understanding of the working principle of the concerned analog to digital converter.

The input voltage is applied at the positive or non-inverting terminal of the comparator while the negative or inverting terminal of the comparator is connected to the output of the digital to analog converter. The output of the binary counter is passed as an input to the DAC.

At the beginning of the conversion, the counter is reset and set to zero. Due to this the output of the digital to analog converter also equals zero. Initially, the given input voltage is higher than the output DAC voltage which satisfies the condition for the comparator’s high state. So, the output of the comparator is high. The comparator is followed by an AND gate which gives clock pulses to the counter for its operation. When the output of the comparator is pulled high, the AND gate applies clock pulses to the counter. Because of this, the counter starts counting. Since the counter keeps on incrementing its count, the output of the digital to analog converter also increases in a staircase fashion. This DAC output is constantly compared with the input voltage.

When Vin > Vdac

As long as the input voltage is greater than the output DAC voltage, the output of the comparator is high and the counter is provided with the clock pulses to perform counting. As a result, the digital to analog output increases slowly in a staircase manner.

When Vin < Vdac

When the digital to analog converter output voltage is greater than the input voltage the state of the comparator becomes low. No pulse will be applied to the counter. Instead, the low output of the comparator goes to the control circuit which in return latches the output of the counter and the counter gets reset. So, we conclude that the latched output is directly proportional to the input voltage. This is the whole procedure and after completion, again the input voltage is sampled and new conversion initiates. The counter resets for every new conversion i.e the counter starts the counting from zero onwards in every conversion.

The analog to digital conversion time is dependent on the magnitude of the input voltage. The greater the applied voltage the more time the analog to digital converter takes for the analog data conversion.

Counter ADC Graphical Representation

The following figure shows the typical conversion pattern of the counter type analog to digital converter:

It is clearly visible that the output of the counter type digital to analog converter increases until it reaches the input voltage. As it crosses the magnitude of the input voltage, the counter resets and the next conversion begins. The green bar determines the time taken by the analog to digital converter for one conversion. We can also see that the part where the input voltage is increased, the conversion time has also increased.

Counter Type AD Conversion Time

It is the time taken by the analog to digital converter to completely transform the analog input to digital output.

The general formula of the maximum conversion time is

Tc(max) = (2^N-1)Tclk

Where N is the number of bits of analog to digital converter and Tclk is the duration of the clock pulse.

Example

For example, if N=4 bits and Tclk=T then maximum conversion time is

Tc(max) = (2^4 -1)T
Tc(max) = 15T

The input voltage becomes equal to the full-scale output voltage of the DAC if all the input bits are 1. For this, the counter takes 2^N-1 clock pulses to reach from all zero bits to all one bits.

This conversion time equation shows its dependency on the resolution and clock frequency. The resolution of this ADC type is based on its DAC. If the bits of digital to analog converter increases, the resolution of the counter type ADC increases which ultimately increases the conversion time. So there is a trade-off between the resolution and conversion time.

An increase in clock frequency can reduce the conversion time but it is also limited by the response time of the components of the analog to digital converter.

Counter type ADC Advantages

It is a simple and easy-to-function integrated circuit.
It is cost-effective.

Counter type ADC Disadvantages

It is a slow analog to digital converter because the converter resets itself for every new conversion.
It is not suitable for high-resolution systems.

Counter Type ADC – Staircase Approximation Analog to Digital Converter