SBAA531 November   2021 ADS8860 , ADS8862 , ADS8881 , ADS9110 , ADS9224R

 

  1.   Trademarks
  2. 1Introduction
  3. 2 Internal Topology of SAR ADC Model
    1. 2.1  Sample and Hold
    2. 2.2  Sample and Hold Timing
    3. 2.3  Reference Transients
    4. 2.4  Bandwidth Modeling
    5. 2.5  Noise Modeling
    6. 2.6  Reference Droop and Reference Noise Errors
    7. 2.7  Gain, Offset, and Input Leakage Modeling
    8. 2.8  Differential input behavior
    9. 2.9  ESD Protection Diodes and Parasitic Capacitance
    10. 2.10 Summary of Parameters
    11. 2.11 Summary of Model Pins
  4. 3Downloading and Using PSpice® Example Projects From Web
    1. 3.1 Selecting the Amplifier and Optimizing the RC Circuit
    2. 3.2 Worst-Case Settling by Adjusting the Reset Capacitor
    3. 3.3 Verification of Reference Droop
    4. 3.4 System Noise Verification
    5. 3.5 Gain, Offset, and Input Leakage Verification
  5. 4Summary

2.10 Summary of Parameters

Table 2-1 lists a short summary for each parameter. For more details on the parameters and internal model schematics see the previous sections of this application report.

Table 2-1 Parameter Summary
ParameterDefinition
CSHSample and hold input capacitor. Typically, this internal capacitor is found in the Input Sampling Stage Equivalent Circuit section of the data sheet and also may be listed in the data sheet table. Typical values for this capacitor range from 5 pF to 100 pF.
RSHParasitic resistance of sample and hold switch. Typically, this internal resistor is found in the Input Sampling Stage Equivalent Circuit section of the data sheet and also may be listed in the data sheet table. Typical values for this resistor range from 10 Ω to 200 Ω.
CDIOParasitic capacitance of ESD diodes which is often in the Input Sampling Stage Equivalent Circuit section of the data sheet. This capacitance is typically 4 pF.
CRESSample and hold reset capacitance. This capacitor is used to reset the sample and hold capacitor at the end of the conversion phase and it emulates the droop seen at the end of the conversion phase. Typically, this is set to 10% of the sample and hold capacitor.
TCONVConversion phase time is the amount of time it takes for the internal conversion to complete. This parameter is found in the data sheet table. Always use the largest time for this parameter as this gives a worst case for the acquisition period.
VREFThe ideal value for the voltage reference. If the Vref_ideal pin is not connected, it will float to this value. This parameter is also the specified reference voltage at which the IREF average current is defined.
FSAMPLThe sampling rate at which the IREF average is defined.
NThe resolution of the device which is used to generate N current transient pulses on the reference during the conversion cycle.

IREF

The average value of the reference current. Typically, this value is in the hundreds of microamps, whereas the transient amplitude is in milliamps. This parameter works in conjunction with VREF, FSAMPL, and N to set the reference transients. For example, the ADS8860 is a 16-bit device with A 5-V reference that draws an average of 300 μA at 1 MHz. Its parameters are: IREF = 300U, N = 16, FSAMP = 1 Meg, and VREF = 5 V.

IREF_ONIREF_ON turns on or off the voltage reference transients. IREF_ON = 1 turns on the transients, and IREF_OFF turns off the transients.
ILThe input leakage current of the ADC analog input pins. This parameter is found in the data sheet table. Typical values for this current range from 1 pA to 1 μA.
EGThe gain error specification as a percentage of full-scale range.
EOThe offset error specification in volts. Typical values range from microvolts to millivolts.
NLFThe flicker noise density in nV/√Hz specified as frequency FLW. If the device does not have a specification for flicker noise, set this number equal to NVR and set FLW to a low value (0.01 Hz).
NVRThe broadband noise density nV/√Hz. Derive this value based on the SNR or transition noise of the device (see section Section 2.5).
FLWThe frequency that the flicker noise density (NLF) is defined at in Hertz.
CREFThe reference capacitance. This capacitance sets the cutoff of the filter that the reference noise is applied to. Typically, this value is set to 0.25 CSH.