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Post by Glovisol » Mon Jan 19, 2015 6:35 pm

Input pad calculator.xls
This simple EXCEL file will compute any pad
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What follows may be obvious to those used to software oscilloscopes, but may be helpful to oldtimers who used cathode ray types all their life.

Comparing the DrDAQ input range to that of previous generation conventional graticule oscilloscopes, one of the most obvious differences is the limitation on the absolute input voltage level.

A conventional graticule oscilloscope input sensitivity is given in V/cm. The highest range on the input selector would be 20V/cm (or even 50V/cm) and on a 10 cm graticule you could read +/- 100V or 200Vpp. With a 10:1 probe (provide it was rated for it) you could read 2,000Vpp.

By comparison the DrDAQ scope input offers the highest range of +/- 10V (total scale, NOT per cm) and this can be a problem when handling higher voltages. This difficulty is compounded when using the EXT. 1-2-3 inputs, which are limited to 2.5 V. Furthermore, when the maximum rated value of the input is exceeded, the DrDAQ internal circuitry saturates and if one pays no attention to the RED ALARM, one can obtain false readings.

This problem can be easily fixed by means of resistor PADS and/or commercial probes. The Picoscope feature of letting the operator define his own probe is extremely helpful in this respect: when a probe is defined and used, the software will automatically adapt the display scale accordingly (but the maximum value at the input connector will never change!)

The standard, commercially available BNC probe is the switchable 1:1 to 10:1 probe. This can be used on the scope input, bringing the highest range to +/- 100 V.

The enclosed EXCEL file shows pads that can be used to accommodate various higher voltage levels both at the scope input and at the EXT. inputs, provided the Buffer is used for these, in order to eliminate the effect of the pull-ups. The pads have been calculated to keep the input impedance at 100 KOhm or above. By changing values in the EXCEL file, any pad can be quickly calculated and even higher input impedances achieved.

R1 is the resistor parallel between input and ground; R2 is the series resistor.

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