Analog to digital converter – Terminology
Today in this series of discussion we will have a look at different terminology that is used while discussing Analog to digital converters. Different concepts of this terminology will be discussed in more detail in my further posts.
1. Operating Input voltage range –
Operating Input Voltage Range is the maximum input voltage range including common voltage range that is allowed on any channel of ADC. Beyond this range any voltage input will saturate the device & may damage the ADC. While selecting any ADC, this is a very important criteria.
2. Differential Input Voltage range –
Differential Input range is the maximum, that is full scale differential input range that can be applied to differential input channels of ADC. All the ADC having differential input channels generally have PGA ( Programmable Gain Amplifier). This PGA amplifies the input voltage so that we can measure the input voltage more precisely. The amplification may be 1,2,4,8,16,32,64,or 128 times the input value. But here we have to see that the amplified value should not be more than its full scale operating range. So this range will vary as per the gain of PGA of each ADC.
3. Dynamic range –
Dynamic range is the ratio of rms value of the full scale signal to the total rms noise measured with the inputs shorted together. This value is expressed in decibels.
4. Common Mode Rejection Ratio (CMRR) –
Common Mode Rejection Ratio is ratio of the amplitude of signal referred to input in the converted result to the amplitude of modulation common to a pair of inputs and is expressed in Decibels ( dB). Put simply it is the measure of ability of any ADC to reject signals such as power line noise, common to both inputs. Higher the CMRR the better.
5. Transient Response –
Transient Response is the measure of time required for the ADC to properly acquire the input after a full scale step function is applied to the system. This is the measure of how fast the adc reacts to any kind of transients at the inputs.
6. Signal to Noise Ratio ( SNR) –
This is a very important factor while selecting any ADC. It is the ratio of rms value of actual input signal to sum of all other spectral components below the Nyquist frequency, excluding harmonics & dc. It is expressed in Decibels.
7. Signal to noise and Distortion Ratio (SINAD) –
It is the ratio of rms value of actual input signal to sum of all other spectral components below the Nyquist frequency,including harmonics but excluding dc. This is measured ratio of signal to ( noise + distortion) at the output of ADC. The theoretical signal to (noise + distortion) ratio for an ideal N bit converter with a sine wave input is given by
Signal-to-(Noise + Distortion)=(6.02N + 1.76) dB
It is expressed in Decibels.
8. Effective Number of Bits (ENOB)-
It is a measure of performance. The effective number of bits can be calculated directly from its measured SNR as follow
N= (SNR – 1.76)/6.02.
9. Total Harmonic Distortion (THD)-
Total Harmonic Dsitortion is the ratio of rms sum of the first five harmonic components to the rms value of a full scale input signal. This value is expressed in decibels.
10. Channel to channel cross talk –
This ( also known as channel to channel isolation) is a measure of level of crosstalk between any channel and all other channels. The crosstalk is measured by applying a dc input to the channel under test and applying a full scale, 10 KHz sine wave signal to all other channels. The crosstalk is the amount of signal that leaks into the test channel and is expressed in decibels.
11. Integral Non-linearity error (INL error) –
INL refers to the deviation of each individual code from negative full scale to positive full scale. This is the measure of accuracy to show how much the measured value follows the actual input value. The point used for negative full scale is 1/2 LSB ( Least Significant Code) before the first code transition. Same way the point used for positive full scale is 1.5 LSB after the last code transition.
12. Differential Non-linearity error –
In ideal ADC the code transitions are 1 LSB apart. That is to say the difference of adjacent codes is minimum 1 LSB. This is an ideal condition. The DNL refers to the maximum deviation from this ideal value. This is expressed in decibels.
13. Gain Error –
Gain error is referred is the deviation expressed in of the actual level of the last transition from the ideal level after the offset error is removed.
14. Offset error –
Offset error is the deviation of the actual MSB ( Most significant Code) transition from the ideal MSB transition point. The ideal MSB transition occurs at
an input level 1/2 LSB above analog ground.
15. Noise-Free resolution –
Noise free resolution also known as flicker free resolution uses the peak-to-peak voltage noise. It is measured in bits and is defined by
Noise free resolution= log 2 [ full scale input voltage range/ ADC peak-to-peak noise ].
16. Effective Resolution –
Effective Resolution in units of bits with the equation –
Effective resolution = log 2 [ full scale input voltage range/ADC RMS noise ].
Later on in this series, we will see how all these terms affect the dynamic performance of any analog to digital converter. The understanding of this terminology is very essential to design a good data acquisition system.