This is a draft cheat sheet. It is a work in progress and is not finished yet.
Successive Approximation Register 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 |
Pros |
Cons |
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 |
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Can be expensive. Therefore, usually limited to 8-bit resolution. |
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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. |
Pros |
Cons |
Only two resistor values (R and 2R) |
Lower conversion speed than binary weighted DAC |
Does not require high precision resistors |
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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.
Resolution
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
Example
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 |
Pros |
Cons |
Very Fast |
Needs many parts (255 comparators for 8-bit ADC) |
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Expensive |
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Large power consumption |
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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 |
Pros |
Cons |
High resolution |
Slow due to oversampling |
No precision external components needed |
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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. |
Pros |
Cons |
Input signal is averaged |
Slow |
Greater noise immunity than other ADC types |
High precision external components required to achieve accuracy |
High accuracy |
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Successive Approximation Register ADC |
Sets MSB |
Converts MSB to analog using DAC |
Compares guess to input |
Set bit |
Test next bit |
Pros |
Cons |
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
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