What is the basic phenomenon behind ADC?

psrr on enob

what is the basic phenomenon behind ADC

Re: adc

"the basic phenomena" about ADC are:
- converts continuous voltage(or current) into a set of discrete (discontinuous) values
- time also becomes "granular" since sampling the input analogue voltage is done with a certain clock.(time interval)

It sounds like a loss of information,but actually this is the only way of interfacing analogue and digital world.

Re: adc

hi..
u can check some of the conversion algorithms..
like sigma delta.... SAR... and others also..

do u want to implement it in FPGA or ur asking about analog VLSI ??

jay

Re: adc


adc convertor converts continous voltage to discrete levels & then does some sort of coding(BCD, Binary,etc) to make it ready to use.

a ADC consists of a dac(digital to analogue coverter) yes , it is true.
one of the most used methods use a comparater,counter and a DAC.

The output of the comparater is fed to an counter clock i/p.initially the DAC is fed by all Zeroes. the anologue equvivalent is fed to the comparater. this compares the i/p signal & generates a high o/p if the i/p is greater than the value in the dac. the o/p of the comparater is fed to the counter clock which in turn increases the value fed to the DAC. this process is repeated until the digital equivalent is obtained. this process is carried to such high speeds that u won't know the difference.


thier are other methods too. this one of the popular


hope this helps u

Re: adc

visit the following link to get the proper answer online 24 hrs a day...

www.technocrafts.com/forum/default.asp 8O

adc

there are many methods for ADC. you can visit the followin references
:basic concepts of data converters,by B.Razavi

Re: adc

Most of the signals directly encountered in science and engineering are continuous: light intensity
that changes with distance; voltage that varies over time; a chemical reaction rate that depends
on temperature, etc. Analog-to-Digital Conversion (ADC) and Digital-to-Analog Conversion
(DAC) are the processes that allow digital computers to interact with these everyday signals.
Digital information is different from its continuous counterpart in two important respects: it is
sampled, and it is quantized. Both of these restrict how much information a digital signal can
contain. This chapter is about information management: understanding what information you
need to retain, and what information you can afford to lose. In turn, this dictates the selection
of the sampling frequency, number of bits, and type of analog filtering needed for converting
between the analog and digital realms.

Re: adc

The main principles involved on ADC are:
Anti-aliasing filtering, samplig & hold, and quantization. There are many books that explains widely these steps.

Re: adc

Let me just add my knowledge to this discussion...

There is no phenomena in A to D Convertion!!! A convertion is a human innovation to make the analog world more comprehensive to human and their mechanic/electronic applications.

Nevertheless, we can't explain the ADC saying that ADC is using a DAC...

In terms of a mechanic example here is what an ADC convertion is:

We don't know the weight of a iron ball is.
We take a known weighted iron ball ( say 1Kgr).
We put the unknown ball on a divise that can hold one ball from the one side and the other ball on the other side ( say a wooden board with a stone undernith just in the middle of the board).
If 1K is less than the iron ball then the board want move . If 1K is more than the iron ball the board will lean on the oter side. If 1K is equal to the iron ball the board will level.
That was our first approximation. We compared an unknown quantity with a know one and we tried to approximate the unknown value. We Digitized with digits of 1K each the unknown weight.

This is how ADC convertion works.

I hope that was helpfull to understand the basik principle ( not phenomena) of ADC. If you need more, always at your disposal !
D.

Re: adc

refer this you will get answer of your question

Re: adc

do you have some implmented circuits for A/D conversion with serial output

Glossory for A/d


Absolute Maximum Ratings – Voltages and currents beyond which a device may not be stressed without danger of damaging or
destroying the device. The device is NOT guaranteed to work when stressed at or near its absolute maximum ratings..
A/D – See ADC.
A/D Converter – See ADC.
A.C. Termination – Transmission line termination technique where a series RC is used at the receiving end of a transmission line.
A.C. Termination – Transmission line termination technique where a series RC is used at the receiving end of a transmission line.
ADC – Analog-to-Digital Converter. A device or circuit used to convert analog information to digital words.
ADC10D040 – A 10-bit, 40 Msps (Megasample per second) ADC
ADC12040 – A 12-bit, 40 Msps (Megasample per second) ADC
ADC14080 – A 14-bit, 80 Msps (Megasample per second) ADC
Aliasing – Conversion of an input frequency to another frequency as a result of the conversion process. The output frequency of
an ADC can never exceed ½ the sampling frequency of the ADC without this aliasing. When the input frequency does exceed
½ the sampling frequency, the output frequency becomes the absolute value of [INT(fIN/fS +0.5) * fS – fIN ].
Characteristic Impedance - The impedance a transmission line such that, when driven by a circuit with that output impedance, the
line appears to be of infinite length such that it will have no standing waves, no reflections from the end and a constant ratio of
voltage to current at a given frequency at every point on the line.
DAC – Digital-to-Analog Converter. A device or circuit used to convert digital words into analog voltages or currents.
Director – The shorter elements of a “Yagi” antenna that directs energy toward the driven element.
DLE – Differential Linearity Error. Same as DNL.
DNL – Differential Non-Linearity. The measure of the maximum deviation from the ideal step size of 1.00 LSB.
ENOB – Effective Number Of Bits. A specification that helps to quantify dynamic performance. ENOB says that the converter
performs as if it were a theoretically perfect converter with a resolution of ENOB. That is, an ENOB of 7.4 says that the
converter performs, as far as SINAD is concerned, as if it were a perfectly ideal ADC with a resolution of 7.4 bits (assuming you
could have fractional bits). The idea behind ENOB comes from the fact that the absolutely perfect ADC has an SNR that
comes only from quantization noise and has absolutely no distortion. When this is the case, SINAD is then equal to SNR. Since
SNR of the absolutely perfect ADC is SNR = 6.02 * n +1.76, where “n” is the number of ADC output data bits, SINAD = SNR for
a perfect converter, so SINAD = 6.02 * n + 1.76 and n = (SINAD – 1.76) / 6.02 and we say that ENOB = (SINAD – 1.76) / 6.02 .
FFT – Fast Fourier Transform. The FFT is a mathematical operation that converts signals between the time and frequency
domains. We generally call the frequency domain (amplitude vs.. frequency) plot an FFT.
EMI/RFI – Electromagnetic Interference/Radio Frequency Interference. This is the radiation of EM (electromagnetic) energy that
may interfere with other circuits and systems.
FR-4 – A glass epoxy printed circuit board material of woven glass cloth construction laminate with an epoxy resin binder.
Full-Scale Input Swing – The difference between the maximum and minimum input voltages that will produce a valid ADC output
without going over- or under-range.
Gain Error - The error in the slope of the ADC transfer characteristic. It is the difference in the actual and ideal full scale input
range values.
IMD – Intermodulation Distortion. This is the creation of new spectral components that result from two or more input frequencies
modulating each other when the circuit is nonlinear.
ILE – Integral Linearity Error.This is the same as INL.
INL – Integral Non-Linearity. The maximum departure of the ADC transfer curve from the ideal transfer curve. INL is a measure
of how straight is the transfer function curve. There are two popular methods of measuring INL: End Point and Best Fit. The
End-Point method is the most conservative, while the Best Fit method gives lower (better-looking) values. National uses the
End Point method.
Input Dynamic Range – For an ADC, the range of voltages that can be applied to the input without going under or over range.
Input Offset – The difference between the input value of 1.0 LSB and the input voltage that causes the ADC output code to
transition from zero to the first code.
Input Offset Error – The difference between the ideal input value of 0.5 LSB and the input voltage that causes the ADC output
code to transition from zero to the first code.
Jitter – The variation in the timing of a signal’s rising or falling edge. It can be specified as cycle-to-cycle or long term.
Loop Area – The area between the conductors of outgoing and return currents.
LSB – Least Significant Bit. The bit that has the least weight.
Missing Codes – Those ADC codes that never appear at the ADC output. These codes can not be obtained with any input value.
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ABCs of ADCs
Authored by: Nicholas “Nick” Gray
Copyright Ó 2003 National Semiconductor Corporation
All rights reserved
69
Nyquist Rate –The minimum sampling rate (or frequency) needed to prevent frequency aliasing.
Nyquist Frequency –The maximum input frequency beyond which frequency aliasing results.
Offset Error – This is the same as Input Offset Error.
PC Board – Printed Circuit Board.
PCB – Printed Circuit Board.
Proximity Effect – The phenomenon whereby outgoing and return currents want to flow close to each other.
PSRR – Power Supply Rejection Ratio. A measure of how well a circuit rejects a signal on its power supply. There are two ways
to specify PSRR, the most common of which is to specify the change in one parameter when the d.c. value of the power
supply is changed. That is, one value of d.c. voltage is applied to the supply pins and the selected parameter (e.g. offset error)
is measured. Then another d.c. voltage is applied to the supply pins and the same parameter is again measured. The extent to
which the selected parameter does not change when the supply voltage is changed is the d.c. PSRR. This tells us nothing
about how well an a.c. signal, such as noise, on the supply line will be rejected by the device.
The other method is the specify how an a.c. signal on the power supply will affect the output of the device. National specifies
both methods for most of our ADCs. This provides the all-important a.c. PSRR, but also provides d.c. PSRR that may be
compared with competition. Note, however, that the two readings have no relationship to each other.
Quantization – The process of dividing a range of analog voltages or currents into smaller “quanta” (smaller range of voltages or
currents) such that each quanta is represented by a single digital code.
Quantization Error – The error introduced as a result of the quantization process. The amount of this error is a function of the
resolution of the quantizer. By definition, the quantization error of an ADC is ½ LSB.
Quantization Noise – The noise at the ADC output that is caused by the quantization process. It is defined as 20 * log (2(n-1) *
sqrt(6) ), or about 6.02 * n + 1.76 dB, where “n” is the number of output bits of the ADC.
Quantizer – A circuit that carries out the quantization process. Another name for an Analog-to-Digital Converter.
Reference Voltage – For an ADC, the reference voltage is the voltage against which the analog input or an ADC is compared to
determine the ADC output code. For a DAC, the reference voltage is multiplied with the ratio of the DAC input code to its (fullscale
code + 1) to determine its analog output.
Reflector – The longer elements of a “Yagi” antenna that reflect energy back to the driven element.
Resolution – A measure of how well the ADC input is “resolved”, or how well the value of an LSB represents the analog input.
Resolution is usually expressed in bits, and then indicates the number of bits available in the ADC output word.
The number of discrete output states or values of an ADC or a DAC, Can also be expressed in the number of digital bits in the
output (for ADCs) or the input (for DACs).
Sampling Noise – The inherent noise of an ADC that comes from the steps in the transfer function.
Scale Factor – The effective multiplier of the analog reference voltage input to an ADC or DAC. This value is usually one, but can
be any whole or fractional number.
Series Termination - Adding a resistor in series with a transmission line such that the driver output impedance plus the resistance
of this external resistor is equal to the characteristic impedance of the transmission line.
S/(N+D) – Signal-to-Noise Plus Distortion. See SINAD.
SINAD – Signal-to-Noise And Distortion ratio.A combination of the SNR and THD specifications, SINAD is defined as the rms
value of the fundamental signal to the rms value of all of the other spectral components below half the clock frequency,
including harmonics but excluding d.c. SINAD can be calculated from SNR and THD. Because it compares all undesired
frequency components at the output with desired frequency. It is an overall measure of the dynamic performance of the ADC.
SINAD is also known as SNDR, S/(N+D) and Signal-to-Noise Plus Distortion.
Skin Effect – The phenomenon by which high frequency current flow is restricted to the surface,or skin, of a conductor.
SNDR – Signal-to-Noise And Distortion Ratio. See SINAD.
SNR – Signal-to-Noise Ratio. The ratio of the power in the signal to the power in all other spectral components below ½ the
sampling frequency, excluding harmonics and d.c.
Split Ground Plane – Concept where analog and digital grounds are in a single PCB layer and only connected at a single point.
Substrate – The base semiconductor material upon with solid state devices are built. The substrate is resistive with a resistance
that is on the order of a few Ohms.
THD – Total Harmonic Distortion. The ratio of the rms total of a specified number of harmonic components to the rms value of the
output signal. National uses the first nine harmonics (f2 through f10).
Through Hole – The hole that goes through a printed circuit board to connect together lines and/or planes in two or more layers
VREF – See “Reference Voltage”.
ZO – The characteristic impedance of a transmission line

Re: adc

try these
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