window detector working

Discussion in 'Homework Help' started by swty_todd, Apr 3, 2009.

1. swty_todd Thread Starter Active Member

Aug 3, 2008
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0

I am having a problem understanding the window detector ckt using a high precision comaparator LM1414. Suppose Vut =7,Vlt=2 and if Vin=8,then C1 will be negatively saturated & C2 will be positively saturated , when I think both should be negatively saturated.

Searching on the net I came across this circuit of Window Detector using IC741 & the difference between the 2 ckts is that 741 ckt contains 2 extra diodes.I understood the working of the ckt using 741 so I was wondering if they forgot to place diodes in the LM1414 ckt or the diodes already present in the LM1414 or is it something else that I cant see?

2. t_n_k AAC Fanatic!

Mar 6, 2009
5,448
783
This is a curious one.

Say there were diodes added with the load resistor to ground as shown.

If the two cathodes were joined at the output node, then the output would be high for any input condition. Behaves a bit like an OR function. Not much use.

If the two anodes were joined at the output node, then the output would always be low. The comparator low output state could never be less than 0V.

With the load resistor tied to the positive rail and diodes added as in the alternative LM741 based cct. then that would work as intended. A bit like an AND function.

Maybe there is something happening inside at the comparator output which allows the circuit to work ....???

3. beenthere Retired Moderator

Apr 20, 2004
15,815
283
Both comparators have high outputs only when the applied voltage is between the upper and lower limits. Applying 8 volts is above the upper limit, so C1's output will go low. C2's output will be high, as the voltage is above 2 volts, but it doesn't matter, as either one going low will pull the other down as well.

When you tie outputs together as in the LM1414, any low will always pull the collective output down. In many cases, the devices used in applications like this have open collector outputs, so they can be pulled up with a single resistor. The LM1414 is designed for use as a window comparator, so its output stage does not suffer damage by use in a wired OR configuration.

The 741 op amps are not comparators. Get the data sheets on the LM1414 and the uA741 to see the differences. Those diodes protect the output stage of the one 741 with a high output by isolating it from the other low condition. The op amp's output would suffer overheating otherwise.