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ADC and DAC Cheat Sheet (DRAFT) by

This is a draft cheat sheet. It is a work in progress and is not finished yet.

Flash ADC

Sigma-­Delta ADC

Successive Approx­imation Register ADC

Dual Slope ADC

Digital to Analog Conversion

A digital to analog converter (DAC) converts a digital signal to an analog voltage or current output.

Types of DAC

Binary Weighted Resistor
Utilizes a summing op-amp circuit
Weighted resistors are used to distin­guish each bit from the most signif­icant to the least signif­icant
Transi­stors are used to switch between Vref and ground (bit high or low)
Assume Ideal Op-amp
No current into op-amp
Virtual ground at inverting input
Vout= -IRf
Simple Constr­uct­ion­/An­alysis
Requires large range of resistors (2000:1 for 12-bit DAC) with necessary high precision for low resistors
Fast Conversion
Requires low switch resist­ances in transi­stors
Can be expensive. Therefore, usually limited to 8-bit resolu­tion.
R-2R Ladder
If the bit is high, the corres­ponding switch is connected to the inverting input of the op-amp.
If the bit is low, the corres­ponding switch is connected to ground.
Only two resistor values (R and 2R)
Lower conversion speed than binary weighted DAC
Does not require high precision resistors

Binary Weigthed Resistor

R-2R Ladder


Analog to Digital Conversion

It is an electronic process in which a contin­uously variable (analog) signal is changed, without altering its essential content, into a multi-­level (digital) signal.

Sample and Hold Circuit


The resolution of the converter indicates the number of discrete values it can produce over the range of analog values.

The resolution determines the magnitude of the quanti­zation error and therefore determines the maximum possible average signal to noise ratio for an ideal ADC

ADC Value Calcul­ation

For an N-bit ADC, the digital repres­ent­ation depends on Number of Bits and Reference values


Given a half wave input signal:
x(t) = Acos(t), A = 5V
Full scale measur­ement rang = 0 to 5 volts
ADC resolution is 8 bits:
28 = 256 quanti­zation levels (codes)
ADC voltage resolu­tion,
Q = (5 V − 0 V) / 256
= 5 V / 256 ≈ 0.0195 V
Q ≈ 19.5 mV.

Common ADC Types

Flash ADC
“parallel A/D”
Uses a series of compar­ators
Each comparator compares Vin to a different reference voltage, starting w/ Vref = 1/2 lsb
Very Fast
Needs many parts (255 compar­ators for 8-bit ADC)
Large power consum­ption
Sigma-­Delta ADC
Oversa­mpled input signal goes in the integrator
Output of integr­ation is compared to GND
Iterates to produce a serial bitstream
Output is serial bit stream with # of 1’s propor­tional to Vin
High resolution
Slow due to oversa­mpling
No precision external components needed
Dual-Slope ADC
The sampled signal charges a capacitor for a fixed amount of time
By integr­ating over time, noise integrates out of the conver­sion.
Then the ADC discharges the capacitor at a fixed rate while a counter counts the ADC's output bits.
A longer discharge time results in a higher count.
Input signal is averaged
Greater noise immunity than other ADC types
High precision external components required to achieve accuracy
High accuracy
Successive Approx­imation Register ADC
Sets MSB
Converts MSB to analog using DAC
Compares guess to input
Set bit
Test next bit
Capable of high speed
Higher resolution successive approx­imation ADCs will be slower
Medium accuracy compared to other ADC types
Speed limited ~5Msps
Good tradeoff between speed and cost
Merge columns in Pros and Cons are considered to be in Pros' column

ADC Types Comparison