Other Parts Discussed in Thread: ADS1278-SP, ADS1282
Hello,
I have a couple of questions about how to interpert the datas from the ADS1282-SP, ENOB and Error Budget Analysis.
First of all, I have read all documentation on training.ti.com/adc-noise
I have read this presentation (btw, it's really help me to separate the type of errors introduced by the ADC).
In our design, we have a Vsignal_diff = 50mV going in differential into an ADS1282S-SP. This signal varies slowly in the time (DC signal).
From your table bellow, I should only consider (because i have a dc signal at the input of the ADC) : the Effective resolution, Noise-free resolution and Noise-Free counts to calculate the resolution i can achieved?
We have VREF= VREP-VREFN = 5-0V = 5V. We plan to use only the SINC filter without the FIR filter at 128kSPS.
FSR = VREF/PGA = 5/1 = 5V.
We can calculate the Resolution system:
Resolution Loss:
Log2(%Utilization) = log2(System FSR/ADC FSR) = log2 (50mV/5V) = 6.7bits
To calculate the Effective Resolution (bits), i don't know which SNR to take. The only SNR provided in the datasheet is for High Resolution at 4kSPS (see page 17). I tried to extrapolate (yellow values) the SNR to higher Sampling Rates and at diffirent PGA Gains. If I use the extrapolate values of the bellow table, I got a SNR equal to 102dB at 128kSPS at PGA =1.
PGA (SNR Ratio dB) | |||||||
SPS | 1 | 64 | |||||
250 | 130 | 114 | |||||
500 | 127 | 111 | |||||
1000 | 124 | 108 | |||||
2000 | 121 | 106 | |||||
4000 | 118 | 103 | |||||
8000 | 115 | 100 | |||||
16000 | 111 | 97 | |||||
32000 | 108 | 94 | |||||
64000 | 105 | 91 | |||||
128000 | 102 | 88 |
FSR_rms = (VREFP-VREFN)/(2 x sqrt(2) x PGA) = 1.7677 Vrms (formula from the datasheet, if PGA = 1)
I can calculate the Vn,RMS = FSR_rms / 10^(SNR/20) = 1.7677 / (10^(102 dB/20)) = 14.04uVrms (by using the extrapolate table above)
Effective Resolution = Log2(System FSR/Vn,RMS) = Log2(50mV/14.04uVrms) = 11.79bits
System Resolution = Effective Resolution-Resolution Loss = 11.79 - 6.7 = 5.1bits (This number represents the actual resolution which i can achieve in my system without considering errors like INL, Offset errors, Gain errors, Gain error drift and Offset error drift?)
However, if i want to use a PGA equal to 64,
log2(System FSR x 64 / ADC FSR) = log2 (50mV x 64 /5V) = -0.644 bits
FSR_rms = (VREFP-VREFN)/(2 x sqrt(2) x PGA) = 0.02762 Vrms
Vn,RMS = FSR_rms / 10^(SNR/20) = 0.02762V/ (10^(88 dB/20)) = 1.0996uVrms (88dB comes from the table above which i extrapolate)
Effective Resolution = Log2(System FSR/Vn,RMS) = Log2(64 x 50mV/1.0996uVrms ) = 21.47 bits
System Resolution = Effective Resolution-Resolution Loss = 21.47- 0.644bits = 20.82bits (This number represent the actual resolution which i can achieve in my system without considering errors like INL, Offset errors, Gain errors, Gain error drift and Offset error drift?)
If I'm taking the last case with a PGA = 64, my resolution system is 20.82bits. To calculate my error budget (INL, Offset Error, Drift, Gain Error, Gain error drift, Noise RTI), should I take 31 bits or 20.82 bits to calculate the ERROR in term of LSB?
If i'm taking 31 bits, 1 LSB voltage is 2.32nV and 0.000465 LSB (ppm) and i got these errors with a gain of 64, SNR = 88dB
Source of Error | VALUE | ERROR (%) | ERROR (LSB) | ERROR (PPM) | LSB (PPM) | |
DNL | DNL | 0 | 0.000 | 0.000 | 0.00 | 0 |
QUANTIZATION ERROR | QUANTIZATION ERROR | 0 | 0.000 | 0 | 0.00 | 0 |
NOISE RTI | NOISE RTI (nVp-p) | 3110.21 | 0.000062 | 1336 | 0.62 | 1336 |
INL | INL (%FSR) | 0.000001953 | 0.000001953 | 42 | 0.02 | 42 |
OFFSET | Offset Error (uV) @PGA = 1, -38 | 38 | 0.049 | 1044536 | 486.40 | 1044536 |
Offset Drift (uV/Degrees) @PGA = 1, -0.01 | 0.01 | 0.001 | 16493 | 7.68 | 16493 | |
GAIN | Gain Error (%), -1.05 | 1.05 | 1.050 | 22548578 | 6720.00 | 14431090 |
Gain Error Drift (ppm/Degrees) @PGA=1, -1 | 1 | 0.004 | 82463 | 38.40 | 82463 |
ERROR (%) | ERROR (LSB) | ERROR (PPM) | LSB (PPM) | |
TOTAL ERROR (WCA) | 1.38 | 29706402 | 10053 | 21588914 |
TOTAL ERROR (RSS) | 1.09 | 23360054 | 7296 | 15668762 |
BITS LOSSES (WCA) | 24.36 | |||
BITS LOSSES (RSS) | 23.90 |
I'm loosing 23.90 bits on 31 bits. So, my system Resolution is 7.044 bits.
If i'm taking 20.82 bits (from the previous part), 1 LSB voltage is 2.701uV and 0.540 LSB (ppm) and i got these errors with a gain of 64, SNR = 88dB
Source of Error | VALUE | ERROR (%) | ERROR (LSB) | ERROR (PPM) | LSB (PPM) | |
DNL | DNL | 0 | 0.000 | 0.000 | 0.00 | 0 |
QUANTIZATION ERROR | QUANTIZATION ERROR | 0 | 0.000 | 0 | 0.00 | 0 |
NOISE RTI | NOISE RTI (nVp-p) | 3110.21 | 0.000062 | 1 | 0.62 | 1 |
INL | INL (%FSR) | 0.000001953 | 0.000001953 | 0 | 0.02 | 0 |
OFFSET | Offset Error (uV) @PGA = 1, -38 | 38 | 0.049 | 900 | 486.40 | 900 |
Offset Drift (uV/Degrees) @PGA = 1, -0.01 | 0.01 | 0.001 | 14 | 7.68 | 14 | |
GAIN | Gain Error (%), -1.05 | 1.05 | 1.050 | 19437 | 6720.00 | 12440 |
Gain Error Drift (ppm/Degrees) @PGA=1, -1 | 1 | 0.004 | 71 | 38.40 | 71 |
ADC ERROR | ||||
ERROR (%) | ERROR (LSB) | ERROR (PPM) | LSB (PPM) | |
TOTAL ERROR (WCA) | 1.38 | 25607 | 10053 | 18610 |
TOTAL ERROR (RSS) | 1.09 | 20137 | 7296 | 13507 |
BITS LOSSES (WCA) | 14.18 | |||
BITS LOSSES (RSS) | 13.72 |
I'm loosing 13.72 bits on 20.82bits. So my system resolution is 7.1bits. The resolution is 1LSB = VREF (5V) / 2^7.1bits = 0.0364V
Does-it make sense?
Thank for your help