The analog circuitry necessary to precisely resolve picoamperes can challenge even the best designer. At these minute current levels, noisy and nonreproducible circuit topologies are common.
current-to-voltage (I/V) converter that uses an op amp and large feedback resistor is the obvious first design choice to acquire a small current. However, the component errors that this method induces can swamp the measurable current (and can amount to throwing out the baby with the bath water). The feedback resistor must be large (Gigohm) and thus is expensive. In addition, the high gain of this topol ogy amplifies any input stimulus, including external noise, that many times exceeds the current input signal of interest.
A better design topology for overcoming these I/V-converter shortfalls is to use an integrating transimpedance amplifier: Swap the feedback resistor for a capacitor in the I/V converter and add hold and reset switches. Figure 1 configures a monolithic transimpedance amplifier (IC1) as a picoammeter. The circuit’s resolution is 0.1 pA, and its range is ±204.6 pA or ±204.6 nA (with the nanoampere range switched closed). The circuit includes an A/D converter, a math look-up table (EPROM), timing logic, an LCD drive, and a display.
A useful application for this picoammeter is the measurement of large resistors (insulators). By configuring four 9V batteries for a ±18V supply with a subregulated 5V supply using an LM7805, you can float this ammeter on top of a high-voltage source (greater than 100V) to measure picoamperes. Thus, the circuit can safely and accurately measure gigohm and teraohm resistances. These high insulation resistances are common values in pc-board materials such as FR-4, which has an insulation resistance of 12 TV/cm.
IC1 is a self-contained transimpedance amplifier with an internal feedback capacitor, a hold switch, a reset switch, and a precision op amp (laser trimmed to com pensate for offset and drift errors). This amplifier forms the input block for the picoammeter and pinstraps the 30-pF internal feedback capacitor to the noninverting input. This capacitance, along with the integration time set by the 555 timer, scales the I/V output by the transfer equation.
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