Tuesday, May 14, 2013

Circuit Design for Reliability Part 3

Continuing the circuit reliability discussion, we will focus on the importance of component selection and board layout in this Blog. There are three major factors designers need to keep in mind while selecting the components: input bias current, input offset voltage and noise figures.

Let’s say the circuit is for audio applications and the op-amp is TL072 selected for low noise and wide bandwidth, running on supply voltages of +/-12V. The TL072 has a maximum quoted Vos of 10mV. With a gain of 1000, this will be amplified by a closed loop gain to give a dc offset at an output of 10V – which is far too close to the supply rail to leave any headroom to cope with overloads. In fact, the TL072 is likely to saturate at 9-10V anyway with +/-12V power rails.

The significance of input bias current is: it determines the steady-state input impedance of the amplifier. This low input impedance affects the performance and accuracy of the source of signal like sensors.  A perfect amplifier with perfect components would be capable of amplifying an infinitely of small signals to, say, 10V p-p with perfect resolution. The imperfection which prevents it from doing so, is called noise. The noise contribution of the amplifier circuit places a lower limit on the resolution of the desired signal, and designer will need to account for it when working with low-level (sub-millivolt) signals or when the signal-to-noise ratio needs to be high, as in precision amplifiers, audio or video circuits.

Selecting the components requires an in-depth circuit simulation and analysis with sensor model, noise values of the amplifier over bandwidth of the signal, and variation in bias current/offset voltage over temperature range. PNC’s dedicated circuit engineers are experts in circuit analysis and have been supporting Companies and Individuals across the nation.