Development of an in situ calibration method for current-to-voltage converters for high-accuracy SI-traceable low dc current measurements

Abstract

For photocurrent measurements with low uncertainties, wide dynamic range reference current-to-voltage converters and a new converter calibration method have been developed at the National Institute of Standards and Technology (NIST). The high-feedback resistors of a reference converter were in situ calibrated on a high-resistivity, printed circuit board placed in an electrically shielded box electrically isolated from the operational amplifier using jumpers. The feedback resistors, prior to their installation, were characterized, selected and heat treated. The circuit board was cleaned with solvents, and the in situ resistors were calibrated using measurement systems for 10 k to 10 G standard resistors. We demonstrate that dc currents from 1 nA to 100 µA can be measured with uncertainties of 55 × 10 −6 (k = 2) or lower, which are lower in uncertainties than any commercial device by factors of 10 to 30 at the same current setting. The internal (NIST) validations of the reference converter are described.

Gain Calibration of Current-to-Voltage Converters

“…This publication supersedes Ref. [3] and finalizes the uncertainty analysis and description of the calibration service.…”

“…The NIST CVC calibration system consists of a precision current source and a voltmeter [5]. The current source is calibrated using the NIST transfer standard CVC (TS CVC) [3]. The TS CVC is a critical element in the calibration traceability and has a smaller uncertainty than was previously achievable.…”

“…The basic op-amp design and construction of the TS CVC were described in Refs. [3,5]. The feedback resistors are precious-metal-oxide film-type resistors and were selected after characterizing various parameters, such as temperature coefficient, voltage coefficient, and drift, to find the most stable and closest to nominal value resistance.…”

“…However, recent advances in photometric and radiometric applications at the National Institute of Standards and Technology (NIST) have prompted the need for lower uncertainties. This need motivated research to reduce the uncertainty and improve the SI traceability for the calibration of CVCs [1][2][3]. NIST has developed a calibration system to measure the CVC gain and the offset for DC photocurrents [3,4].…”

“…This need motivated research to reduce the uncertainty and improve the SI traceability for the calibration of CVCs [1][2][3]. NIST has developed a calibration system to measure the CVC gain and the offset for DC photocurrents [3,4]. This publication supersedes Ref.…”