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[FAQ] What is the impedance of the device's input pin? Is it CMOS or bipolar device?

Other Parts Discussed in Thread: SN74LVC1G125

FAQ: Logic and Voltage Translation > Input Parameters >> Current FAQ

The impedance of a device's pin can be determined by Ohm's law : R= V/ I. 

Usually, the impedance of the CMOS input pins ( OE, DIR, A, B, Preset, CLR, select pins etc) will be very high in Mega ohms. This can be estimated from the electrical characteristic of the datasheet looking into the input leakage current spec.

For eg: Calculating the input impedance of the SN74LVC1G125 device's OE/ A pins:

the Electrical spec lists the input leakage max of 5uA for 0 to 5.5V on the A or OE pins.

The R can be estimated by R = 5.5V / 5uA = 1.1Mohm. 

Usually, the leakage is in nA for a nominal CMOS device, and hence the input impedance will be in several hundred mega ohm making it optimized for low power operation.

For a bipolar device, however, the input leakage is sometimes in low mA ( as well as the Icc) resulting in the impedance in kilo ohm range. 

The feature section of the page, the functional block diagram in the datasheet indicates the technology of the device( either Bipolar, BiCMOS).

The input impedance calculated from the input leakage electrical spec indicates if the device is CMOS. Almost all new logic devices being developed incorporate CMOS technology which are power efficient.

The updated logic parametric search page lists the input type of the device; either Bipolar, CMOS or others.