Guarded input picoammeter circuit

The circuit employs CA3160 and CA3140 BiMOS operational amplifiers to achieve a full-scale meter deflection of ±3 pA. The CA3140 functions as a 1T0 gain stage, supplying the necessary positive and negative output swing for the meter and the feedback network. Additionally, terminals 2 and 4 of the CA3160 are grounded, allowing its input to operate in a 'guarded mode.'

The circuit design features two key components, the CA3160 and CA3140, which are both BiMOS operational amplifiers known for their high input impedance and low noise characteristics. The CA3140 is configured as a 1T0 gain stage, which is crucial for amplifying the input signal while maintaining stability and precision. This configuration ensures that the output can swing positively and negatively, accommodating the full-scale deflection requirements of the meter, which is calibrated to reflect currents as low as ±3 pA.

In this setup, the CA3160 acts as a differential amplifier with its terminals 2 and 4 connected to ground. This grounding is essential for establishing a reference point and enhancing the circuit's noise immunity. The 'guarded mode' operation of the CA3160 minimizes leakage currents and ensures that the input stage remains unaffected by external noise sources, thereby improving the overall accuracy of the measurement.

The feedback network is designed to maintain the desired gain and stability of the circuit. Proper selection of feedback resistors is critical in determining the gain of the CA3140 stage, ensuring that the output remains within the specified limits for the meter's operation. Furthermore, careful layout and shielding techniques should be employed to mitigate any potential interference that could affect the low-level current readings.

Overall, this circuit configuration is well-suited for applications requiring precise measurement of very low currents, leveraging the unique characteristics of BiMOS technology to achieve high performance in sensitive electronic measurement systems.The circuit utilizes CA3160 and CA3140 BiMOS op amps to provide a full-scale meter deflection of ± 3 pA. The CA3140 serves as an 1T0 gain stage to provide the required plus and minus output swing for the meter and feedback network.

Terminals 2 and 4 of the CA3160 are at ground potential, thus its input is operated in the ' 'guarded mode.'' 🔗 External reference