HP 34970A Using an External Pull-Up, Driving External Switches

Chapter 8 Tutorial

Multifunction Module

Using an External Pull-Up

In general, an external pull-up is required only when you want to set the output “high” value greater than TTL levels. For example, to use a +12V external power supply, the value of the external pull-up resistor is calculated as follows:

Vcc = 12 Vdc

Imax = Iout low x safety factor = 1 mA x 0.5 = 0.5 mA

R =	Vcc	=	12	= 24 kΩ
	I		0.0005
	max

The value of the logic “high” level with the external 24 kΩ pull-up resistor is calculated as follows:

Vhigh	= Vcc x	R external		= 12 x	24 k Ω			= 8.47 Vdc
		R external	+ R internal		24 k Ω	+ 10	k Ω

Driving External Switches

You can use two digital output channels to control an external switch. For example, you can drive the HP 865X series of microwave switches using an external power supply and two digital output channels.

The HP 865A switch provides an internal protection diode. The state of the 2-to-1 multiplexer is changed by setting the appropriate output bit low (0).

Digital Output Channel HP 865X Microwave Switch

24Volt Power Supply

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