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.

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.

High Impedance Sensors (pdf, 241,083 bytes)