AD546 Input current, IB, will contribute an output voltage error, VE1, proportional to the feedback resistance:
tacting the device under test. Rigid Teflon coaxial cable is used to make connections to all high impedance nodes. The use of rigid coax affords immunity to error induced by mechanical vibration and provides an outer conductor for shielding. The entire circuit is enclosed in a grounded metal box.
VE1 = IB × RF The op amp’s input voltage offset will cause an error current through the photodiode’s shunt resistance, RS:
The test apparatus is calibrated without a device under test present. A five minute stabilization period after the power is turned on is required. First, VERR1 and VERR2 are measured. These voltages are the errors caused by offset voltages and leakage currents of the current to voltage converters.
I = VOS/RS The error current will result in an error voltage (VE2) at the amplifier’s output equal to: VE2 = (1 +RF/RS) VOS
VERR1 = 10 (VOSA – IBA × RSa) VERR2 = 10 (VOSB – IBB × RSb)
Given typical values of photodiode shunt resistance (on the order of 109 Ω), RF/RS can be greater than one, especially if a large feedback resistance is used. Also, RF/RS will increase with temperature, as photodiode shunt resistance typically drops by a factor of two for every 10°C rise in temperature. An op amp with low offset voltage and low drift helps maintain accuracy.
Once measured, these errors are subtracted from the readings taken with a device under test present. Amplifier B closes the feedback loop to the device under test, in addition to providing current to voltage conversion. The offset error of the device under test appears as a common-mode signal and does not affect the test measurement. As a result, only the leakage current of the device under test is measured. VA – VERR1 = 10[RSa × IB(+)] VX – VERR2 = 10[RSb × IB(–)] Although a series of devices can be tested after only one calibration measurement, calibration should be updated periodically to compensate for any thermal drift of the currento-voltage converters or changes in the ambient environment. Laboratory results have shown that repeatable measurements within 10 fA can be realized when this apparatus is properly implemented. These results are achieved in part by the design of the circuit, which eliminates relays and other parasitic leakage paths in the high impedance signal lines, and in part by the inherent cancellation of errors through the calibration and measurement procedure. PHOTODIODE INTERFACE
The AD546’s 1 pA current and low input offset voltage make it a good choice for very sensitive photodiode preamps (Figure 39). The photodiode develops a signal current, IS, equal to: IS = R × P where P is light power incident on the diode’s surface in watts and R is the photodiode responsivity in amps/watt. RF converts the signal current to an output voltage: VOUT = RF × IS
Figure 40. Photodiode Preamp DC Error Sources Photodiode Preamp Noise
Noise limits the signal resolution obtainable with the preamp. The output voltage noise divided by the feedback resistance is the minimum current signal that can be detected. This minimum detectable current divided by the responsivity of the photodiode represents the lowest light power that can be detected by the preamp. Noise sources associated with the photodiode, amplifier, and feedback resistance are shown in Figure 41; Figure 42 is the voltage spectral density versus frequency plot of each of the noise source’s contribution to the output voltage noise (circuit parameters in Figure 40 are assumed). Each noise source’s rms contribution to the total output voltage noise is obtained by integrating the square of its spectral density function over frequency. The rms value of the output voltage noise is the square root of the sum of all contributions. Minimizing the total area under these curves will optimize the preamplifier’s resolution for a given bandwidth.
Figure 39. Photodiode Preamp
DC error sources and an equivalent circuit for a small area (0.2 mm square) photodiode are indicated in Figure 40. Figure 41. Photodiode Preamp Noise Sources