Application Notes

LOW INPUT BIAS CURRENT

The AD854x family is a CMOS op-amp family and features very low input bias

current in pA range. The low input bias current allows the amplifiers to be used in

applications with high resistance sources. Care must be taken to minimize PCB Surface

Leakage. See below section on “PCB Surface Leakage” for more details.

PCB SURFACE LEAKAGE

In applications where low input bias current is critical, Printed Circuit Board (PCB)

surface leakage effects need to be considered. Surface leakage is caused by humidity, dust or

other contamination on the board. Under low humidity conditions, a typical resistance

between nearby traces is 1012Ω. A 5V difference would cause 5pA of current to flow, which is

greater than the AD854xA’s input bias current at +25℃ (±1pA, typical). It is recommended

to use multi-layer PCB layout and route the op-amp’s -IN and +IN signal under the PCB

surface.

The effective way to reduce surface leakage is to use a guard ring around sensitive pins

(or traces). The guard ring is biased at the same voltage as the sensitive pin. An example of

this type of layout is shown in Figure 1 for Inverting Gain application.

1. For Non-Inverting Gain and Unity-Gain Buffer:

a) Connect the non-inverting pin (+IN) to the input with a wire that does not touch the

PCB surface.

b) Connect the guard ring to the inverting input pin (-IN). This biases the guard ring to

the Common Mode input voltage.

2. For Inverting Gain and Trans-impedance Gain Amplifiers(convert current to voltage,

such as photo detectors):

a) Connect the guard ring to the non-inverting input pin (+IN). This biases the guard ring

to the same reference voltage as the op-amp (e.g., VS/2 or ground).

b) Connect the inverting pin (-IN) to the input with a wire that does not touch the PCB

surface.

Figure 1 Use a guard ring around sensitive pins

1.1MHz, 85µA, CMOS, RRIO Operational Amplifiers

UMW

AD8541,AD8542,AD8544

R

www.umw-ic.com

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UTD Semiconductor Co.,Limited