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Interactive Design Tools: Operational Amplifiers : Photodiode Preamp Error Budget Tutorial (BETA)

An online tool to illustrate range, gain and accuracy issues
in photodiode preamplifiers.

Opamp:
Instructions | Troubleshooting | Related Information
Examples
  Topology
Positive Supply  
Illumination
IPD
A
Schematic
RF
M ohms
RF'
M ohms
VREF
VBIAS
RL
K ohms
CL
pF
Ideal Opamp
 
Negative Supply  
Application Parameters

Operating Temp., TA °C Operating Frequency, f (<1MHz) Hz
Opamp Input Capacitance, CM pF Additional Input Cap.,
CXTRA ( trace, etc. )
pF
Opamp Diff. Inp. Cap., CD pF Noninverting Bypass Cap., CBP uF
Compensating Cap, CF pF 3dB BW for given RF MHz

Photodiode Parameters

Responsivity & Area A/W
mm2
Max Linear
Current, IPD-MAX
µA
Shunt Resistance,
RPD-SH at 25°C
Mohms RPD-SH at temp. Mohms
Dark Current,
IPD-DARK at 25°C
A IPD-DARK at temp. and bias A ( photoconductive only )
Junction Cap., CPD ,
zero-bias
pF CPD at bias pF

Error Source Specification Approx. Calculation Absolute
DC Error
Drift
DC Error
Gain
Error
Resolution
Error

Resistor Tolerance %   ppm
Resistor Drift, TCR
ppm / °C
Max: TCR × TDIFF     ppm
Temp. difference, TDIFF °C  

Nom. Open Loop Gain, AVOL V/mV   ppm
Min. Open Loop Gain, AVOL-MIN V/mV   ppm
Gain-BW product, GBW Hz   ppm

Input Offset Voltage, VOSI mV   ppm
Input Offset Voltage Drift, VOSI_TC µV / °C   ppm

Bias Current, IB nA   ppm
Bias Current Drift, IB_TC pA / °C   ppm
Offset Current, IOS nA ppm
Offset Current Drift, IOS_TC pA / °C   ppm

Dark Current, IPD-DARK   ppm
Shunt Resistance, RPD-SH   ppm
Output Resistance, RO ohms

Common Mode Rejection Ratio, CMRR dB (inv: (1+1/gain)×) 10 -CMRR/20 ×
| (V++V-)/2 - (VS++VS-)/2 | /
| VIN-VREF |
  ppm
Power Supply Rejection Ratio, PSRR dB (inv: (1+1/gain)×) 10 -PSRR/20 × | ( VS+-VS- ) -
( VS+nom-VS-nom ) | / | VIN-VREF |
ppm
Supply Variability
(ripple+load reg.)
% 10 -PSRR/20 × SUP-VAR ×
( VS+-VS- ) / | VIN-VREF |
  ppm

Noise BW* 0.01 -
Hz
  Noise components ppm   
Voltage noise, VNW
nV/root-Hz
Corner freq   Hz
Current noise, INW
pA/root-Hz
Corner freq   Hz
Total Harmonic Distortion, THD dB 10THD/20    ppm

Total AC error (AC) ppm   ppm ppm
Total DC error ppm   ppm ppm

PPM to uV converter: ppm    uV

Instructions

This calculator has two parts: an annotated schematic at top and a table of contributing error sources at bottom. Opamp parametric data is automatically entered in the appropriate fields by choosing an opamp from the menu at top. Default application parameters have been entered in the fields at top with default photodiode parameters placed in the section below this and above the opamp parametrics (a few application parameters are mixed in with the opamp data). All input data can be manually overridden, however output fields (surrounded by light gray) cannot be changed.

After entering data in a field, hit tab or click "Update" to compute derived values and see node voltages updated on the schematic. If the inputs are out of range an alert will appear. If the combination of inputs causes internal or external output limits to be exceeded, the problem node value will be highlighted in red and an "Out of Range!" message will appear. When this message is present, all node values should be considered invalid. Do not leave fields blank: if you see NaN (Not a Number), this means that insufficient data was entered to compute a value.

This tool uses a highly simplified model of an opamp, and any results must be used with care. In particular please note that calculated errors are highly dependent on which opamp parametric data is used, which is application-specific. For many of the opamps, two sets of numbers are available in the pull-down: typical and "conservative". It is highly unlikely that all worst-case specs would ever be present at the same time in the same part. The designer should always refer to the appropriate datasheet and substitute numbers most appropriate to his/her application. All calculations are approximations with errors displayed and summed in absolute PPM, even though in some scenarios the actual values would be negative (and could offset other errors).

Equations listed in the "Calculation" column are approximate and reflect the worst case between the three buffer choices. Modifications to the equation for particular buffer types are indicated in (). For example (1/2 : noninv) means an additional factor of 1/2 should be used to compute this quantity for noninverting buffers.

* Noise is calculated as an integrated quantity, assuming uniform spectral density over the noise bandwidth given. By default, the noise bandwidth is initialized to the closed-loop bandwidth * π/2 (π/2 adjusts for the equivalent noise bandwidth for single-pole rolloff of white noise) If a smaller signal bandwidth is entered, the assumption is that the output will be filtered to this bandwidth so as to remove out-of-band noise. Please also note that it is assumed the noninverting input resistor is perfectly bypassed and so is not included in the noise calculations.


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Troubleshooting

Netscape 4 is not completely compatible with this tool at present. Opera 6.05 exhibits a bug parsing predefined opamp data as selected from the top pulldown menus. This can be worked around by manually entering data in corrupted fields (typically the noise parameter data is affected).

For further troubleshooting information, please visit our Interactive Tools troubleshooting page.


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Related Information

High Impedance Sensors (pdf, 241,083 bytes)


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