July 17, 1997

Integrator forms picoammeter

Clayton B Grantham, Burr-Brown Corp, Tucson, AZ

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. A 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,

With this circuit, the scale of V_OUT is ­50 mV/pA.

Figure 1 also shows the hold, reset, and integration timing diagram. The 555 timer indirectly drives the hold-switch input, S₁, of IC₁. The timer starts an A/D conversion that causes the converter's STATUS strobe to control S₁. STATUS typically goes high for 20 µsec (25 µsec maximum) during a conversion. IC₁ holds its output while the conversion takes place. The reset-switch input, S₂, of IC₁ is the delayed 555-timer output, which delays resetting IC₁ until the conversion is complete. 35 µsec after the conversion starts (approximately 15 µsec after the conversion is complete), the reset switch closes for 35 µsec to discharge the 30-pF feedback capacitor. When the timer goes high, the reset switch opens and a new integration period begins. The cycle repeats indefinitely.

The 1-kilohm input resistor protects IC₁ from direct connections to a voltage source (infinite current). This resistor current limits IC₁'s input diodes for voltages to ±30V.

The circuit configures the 555 timer as a free-running multivibrator. The 1-µF capacitor and 100 ohm resistor set a discharge time of 70 µsec for the hold and reset period. The 1-Megohm resistor stretches the charge time, thus controlling the integration period of 1.5 sec. The 2-Megohm potentiometer adjusts the gain in the picoampere range, and the 2-kilohm potentiometer adjusts the gain in the nanoampere range.

The A/D converter is a ±10V converter in the 12-bit-width ADS574 emulator mode. This mode samples the input for 4 µsec and then converts it. The 200V offset-adjust potentiometer allows system offset adjustment, including the initial errors of IC₁ and IC₂.

The A/D converter's digital output performs as a look-up-table pointer. This pointer forms the address for the two 8-bit EPROMs. The EPROMs contain the match calculation that transforms a voltage to four-digit BCD picoamperes on the display. One EPROM is for the most significant two digits, and the other is for the least significant two digits. The choice of components and timing scales 0.1 pA to a 12-bit LSB.

The LCD drivers (CD4543) interface the EPROMs with the 41/2-digit LCD display. A simple, 100-Hz RC/inverter oscillator drives the LCD backplane. All signals driving the LCD are either in phase (segment off) with this clock or out of phase (segment on) to comply with the non-dc LCD-drive requirements (30 to 300 Hz). The MINUS and first decimal-point segments are out of phase (segments on), so they're always displayed. The LCD PLUS input interfaces through exclusive-OR gates with the sign bit of the ADS574's most significant bit.

You calibrate the circuit by removing any input current signal and adjusting the offset potentiometer for 000.0. Then, apply a known current that is close to 200 pA (0.02V and 100-Megohm to form a Thevenin current source) and adjust the 2-Megohm potentiometer for a display of 200.0. Repeat this gain adjust for the nanoampere range with a 200-nA calibration source.

Several design changes can improve this circuit. These changes include enhancing the time base with a crystal clock, consolidating the LCD and integrator time bases, and extending the time-base options for a microampere range. (DI #2033)

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