Analog Circuits - PCB surface leakage current

The latest opamps in the market have good offset performance and very low bias currents. The bias current will be of the order of the 1pA and even of the order of fA. Take for example the latest precision opamp LMP7721 from Texas Instruments which has input bias current as low as 3fA. This is very low and requires very good layout design to take advantage of the good offset and drift performance.

                Leakage effect is one of the primary concerns in such circuits. The leakage can be through the PCB crosstalk which can easily draw more current than the input current drawn by the opamp pins. This is more prevalent in the high temperature and high voltage applications. Over a period of time the dust, humidity and other impurities formed over the PCB aggravates the problem and should be taken care with good layout practices. The impurities that we are talking here can be a flux residue which is left over due to improper PCB cleaning techniques. So, the following procedures are must in such critical applications:

1.       Surface coating on the PCB to avoid humidity, dust accumulation

2.       Using high quality dielectric materials

3.       Proper PCB cleaning techniques

4.       Good layout techniques

Example scenario:

Take for example a PCB trace whose impedance w.r.t given trace nearby is 110K and a 3.3V is applied across the main trace. In this case the current passing across that junction would be 30uA. If the input bias current of an opamp is comparable with this current, then definitely the circuit will misbehave. In such cases we have implement the following technique to avoid that leakage:

Have a guard trace around the trace carrying low current. This guard trace must have a potential very close to main trace. The below snapshot shows guard ring recommendation for the LMC6001 Ultra, Ultra-Low Input Current Amplifier.