Renesas Electronics America Inc. © 2012 Renesas Electronics America Inc. All rights reserved. ADC Resolution: Myth and Reality © 2012 Renesas Electronics America Inc. All rights reserved.2 Renesas Technology & Solution Portfolio © 2012 Renesas Electronics America Inc. All rights reserved.3 Microcontroller and Microprocessor Line-up Wide Format LCDs  Industrial & Automotive, 130nm  350µA/MHz, 1µA standby 44 DMIPS, True Low Power Embedded Security, ASSP 165 DMIPS, FPU, DSC 1200 DMIPS, Performance 1200 DMIPS, Superscalar 500 DMIPS, Low Power 165 DMIPS, FPU, DSC 25 DMIPS, Low Power 10 DMIPS, Capacitive Touch  Industrial & Automotive, 150nm  190µA/MHz, 0.3µA standby  Industrial, 90nm  200µA/MHz, 1.6µA deep standby  Automotive & Industrial, 90nm  600µA/MHz, 1.5µA standby  Automotive & Industrial, 65nm  600µA/MHz, 1.5µA standby  Automotive, 40nm  500µA/MHz, 35µA deep standby  Industrial, 40nm  200µA/MHz, 0.3µA deep standby  Industrial, 90nm  1mA/MHz, 100µA standby  Industrial & Automotive, 130nm  144µA/MHz, 0.2µA standby 2010 2012 32-bit8/16-bit © 2012 Renesas Electronics America Inc. All rights reserved.4  Collecting, analyzing and transmitting real-world signals is a major focus of the smart society. Real-world signals are analog, so converting these signals to digital is a key focus for the smart  Understanding the specifications and hidden errors in ADC circuits will enable designs that meet the intended specifications ‘Enabling The Smart Society’ © 2012 Renesas Electronics America Inc. All rights reserved.5 Agenda  What does the “resolution” spec really mean  Some standard converters and resolution  DC accuracy specifications  Review offset, gain, DNL and INL errors  How the ADC is tested  What those errors don’t tell you  AC specifications  SNR  ENOB  System errors and resolution requirements  ADC required accuracy  Reference errors  Source impedance errors © 2012 Renesas Electronics America Inc. All rights reserved.6 Resolution  What does the term resolution mean to you? © 2012 Renesas Electronics America Inc. All rights reserved.7 7 Successive Approximation (SAR) ADC DAC (R2R Ladder) Comparator Vref AVss ADC Register Sample and Hold Circuit Input Analog Mux AN0 AN1 AN2 AN3 AN4 AN5 AN6 AN7 DC Specs primarily define this section of ADC performance © 2012 Renesas Electronics America Inc. All rights reserved.8 8 Slope Converter R GPIO Timer Vref Enable CPU Start Conversion Vcc  Measure value R – Thermistor or sensor  Operation – Stopped • GPIO = L • Timer stopped – Begin conversion • GPIO = Hi-Z • Timer started • Comp out = H – Conversion ends • Vc > Vref • Comp out = L • Timer stops  Timer value is proportional to RC time constant  Resolution? L H Stopped Started L Clock © 2012 Renesas Electronics America Inc. All rights reserved.9 Delta Sigma Converter Vin ∫ ∑ Ref Digital Filter D CK 4V H H 0V 5V +V Result Register © 2012 Renesas Electronics America Inc. All rights reserved.10 Delta Sigma Converter Vin ∫ ∑ Ref Digital Filter D CK Result Register  Oversampling frequency (flip flop clock rate, e.g. RX21A 3.125MHz)  Minimum Conversion time – rate the result register is updated (81.92 uS or 12.2 kHz on RX21A ) – This is based on the decimation factor of the digital filter – Some converters allow reducing decimation factor • Faster conversion • Lower resolution [...]... 3.0V Vdd Vdd  ADC range and resolution  LSB must be < 10 mV  – 0.25% * 2.0V = 10 mV – 10 mV / 3.0V = 1/300 – 9 bit ADC required AVREFP 3V AVREFM MCU A ADCin 0-2 V Vss  Decreasing Vref to 2.5V – 10 mV / 2.5V = 1/250 – 8 bit ADC meets resolution requirement 21 © 2012 Renesas Electronics America Inc All rights reserved A D Understanding the Errors an Example  Assume Vref = 2.56V, 8 Bit ADC (10 mV per... S4 ADC Input Voltage © 2012 Renesas Electronics America Inc All rights reserved Oversampling ADC Transition Voltages 03 x 02 S3 x S1 S2 x 01 x S4 ADC Input Voltage with noise 00 Result is now 04  Oversample 2X results in ½ bit increase resolution  To increase resolution by n bits – Oversample 4n and decimate 2n 13 ADC Input Voltage © 2012 Renesas Electronics America Inc All rights reserved ADC. .. can increase resolution of ADC ADC Transition Voltages 03 02 01 S1 x S1 x S2 x S2 x S3 x S3 x S4 x S4 x ADC Input Voltage 00 Result will be 04 when samples are added Result will still be 04 when samples are added if no noise 11 © 2012 Renesas Electronics America Inc All rights reserved Oversampling  How noise helps oversampling 03 x ADC Transition Voltages S3 x S1 02 S2 x 01 ADC Input Voltage with noise... can calculate the equivalent perfect ADC from equation ENOB = (SNDR -1.76)/ 6.02  The 6.02 term in the divisor converts decibels (a log10 representation) to bits (a log2 representation)  The 1.76 term comes from quantization error in an ideal ADC  86 dB would then have the equivalent resolution of a 14 bit perfect ADC ENOB = log2 [full-scale input voltage range/ (ADC RMS noise × √12)] 18 © 2012 Renesas... Ideal Curve ADC Counts Corrected Curve Absolute Error Real Curve 0V Vfull Scale Offset Error 15 © 2012 Renesas Electronics America Inc All rights reserved Input Voltage AC Specifications  DC testing does not describe dynamic characteristic  Sample and hold errors  AC noise errors  Comparator hysteresis  AC testing method  Input sine wave to ADC input  Perform FFT  Measure SNDR, SNR and/ or Spurious... Clocks  IO port toggles  Sensor and reference error  Accuracy vs drift  Temperature , age and voltage effects  Calibration Vdd Vdd AVREFP Vref  Input system effects AVREFM MCU A ADCin T GPIO Vss A D 27 © 2012 Renesas Electronics America Inc All rights reserved Check Accuracy Conditions  Specification may expect:  MCU in a sleep mode  No IO toggling  Specified ADC clock speed 28 © 2012 Renesas... All rights reserved 30 0 40 Actual Voltage (mV) Understanding the Errors an Example  Can we use a 10 bit ADC with +/- 2 bits INL  Each LSB error = 2.5mV (2.56V / 1024)  Error for 2 LSB = 5 mV  5 mV/2.0V = 0.25%  But there is still an additional ½ LSB quantization error  6.25 mV total error = 0.31%  What about 1 bit of error  Worst case ADC error is 2.5 mV + 1.25 mV  0.1875% error 23 © 2012... Electronics America Inc All rights reserved Understanding the Errors an Example  Assume Vref = 2.56V, 10 Bit ADC (2.5 mV per step) Indicated Voltage (mV) 10 mV 1.251 mV code 01 Output Code 04 03 With 2 LSB error 7.5 mV 02 5 mV 01 2.5 mV 00 0 0 1.2 5 24 2.5 © 2012 Renesas Electronics America Inc All rights reserved 5 7.5 10 Actual Voltage (mV) When is a 16 Bit ADC Not 16 Bit? Specification TUE - Unadjusted... Specification  AC testing does not provide linearity data  DNL and INL do affect SNDR  DNL affects SNR  INL affect THD  Oversampling is still valid and reduces the average noise if Gaussian distribution 19 © 2012 Renesas Electronics America Inc All rights reserved Example 20 © 2012 Renesas Electronics America Inc All rights reserved Understanding the Errors an Example  Requirements  Input = 0V –... reserved c) non-ratiometric d) non-ratiometric Understanding Reference Errors  Vref is a power supply pin +V  Vref powers R2R ladder Zt Vcc Vref Rt T MCU Vm AD Input Rref 31 © 2012 Renesas Electronics America Inc All rights reserved  Treat as power supply pin – Typically . reserved.5 Agenda  What does the resolution spec really mean  Some standard converters and resolution  DC accuracy specifications  Review offset, gain, DNL and INL errors  How the ADC is tested  What. Electronics America Inc. © 2012 Renesas Electronics America Inc. All rights reserved. ADC Resolution: Myth and Reality © 2012 Renesas Electronics America Inc. All rights reserved.2 Renesas Technology. Lower resolution © 2012 Renesas Electronics America Inc. All rights reserved.11 Oversampling  Oversampling can increase resolution of ADC 00 01 ADC Transition Voltages S1 S2 S3 S4 02 03 ADC