Is there a specific reason to connect a diode to ground?

Thread Starter

rahul411

Joined Feb 19, 2018
251
This dc load circuit i got online uses a diode to separate the input supply ground to the circuit ground (V-) but opamp is connected to input ground (GND). All i can think is reverse input protection but 4148 being a signal diode i must be wrong. Put some light on this.
PicsArt_09-30-05.06.49.jpg
 

bertus

Joined Apr 5, 2008
20,752
Hello,

Are there some labels under the thick orange line you have drawn?
It looks like the diode is used to lift the -V 0.7 volts above GND.
Also I would leave the output of IC1B open.

Bertus
 

AnalogKid

Joined Aug 1, 2013
8,589
***IF*** the load power source is completely isolated from the circuit power source (Vcc), then D1 is there to elevate the measured signal with respect to the measurement circuit negative rail. This increases the circuits operating headroom, the difference in voltage between the lowest signal value and the lowest power rail. It is adding a little extra margin to the circuit's input common mode voltage operating range. Interesting, because the LM158/258/358 is known for having an input common mode range that extends *below* its negative rail. It was one of the first opamps to do this, and was a WOW moment back in 1971.

Also, IC1B pin 7 - the output pin of the unused comparator should not be connected to anything. It is an open-collector output, so the connection to GND shouldn't be a problem, but it is bad practice because a totem-pole-output opamp's output might try to go high when its inputs are shorted. Curse that input offset voltage error. Bad habit! Bad, bad habit!

ak
 

Thread Starter

rahul411

Joined Feb 19, 2018
251
***IF*** the load power source is completely isolated from the circuit power source (Vcc), then D1 is there to elevate the measured signal with respect to the measurement circuit negative rail. This increases the circuits operating headroom, the difference in voltage between the lowest signal value and the lowest power rail. It is adding a little extra margin to the circuit's input common mode voltage operating range. Interesting, because the LM158/258/358 is known for having an input common mode range that extends *below* its negative rail. It was one of the first opamps to do this, and was a WOW moment back in 1971.

Also, IC1B pin 7 - the output pin of the unused comparator should not be connected to anything. It is an open-collector output, so the connection to GND shouldn't be a problem, but it is bad practice because a totem-pole-output opamp's output might try to go high when its inputs are shorted. Curse that input offset voltage error. Bad habit! Bad, bad habit!

ak
T
***IF*** the load power source is completely isolated from the circuit power source (Vcc), then D1 is there to elevate the measured signal with respect to the measurement circuit negative rail. This increases the circuits operating headroom, the difference in voltage between the lowest signal value and the lowest power rail. It is adding a little extra margin to the circuit's input common mode voltage operating range. Interesting, because the LM158/258/358 is known for having an input common mode range that extends *below* its negative rail. It was one of the first opamps to do this, and was a WOW moment back in 1971.

Also, IC1B pin 7 - the output pin of the unused comparator should not be connected to anything. It is an open-collector output, so the connection to GND shouldn't be a problem, but it is bad practice because a totem-pole-output opamp's output might try to go high when its inputs are shorted. Curse that input offset voltage error. Bad habit! Bad, bad habit!

ak
Thanks
 
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