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Diff amp for negative input and negative reference input.

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maestroharsh

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Below is a diff amp I am planning to use to read value from a capacitor. I will be using a saw tooth reference voltage and output will be high when vref>Vin else low. Problem is my input voltage is negative dc voltage. Can you suggest me how to change my circuit?


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.param 
+      Lch = 500n
+      Wnsa = 300n
+      Wpsa = 300n

* Differential Amplifier to cleanup the output clock
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.subckt diffAmp in1 in2 o vdd 
Mp1 oSA  obSA vdd   vdd Pch5 L=Lch W=Wpsa AS='1.5*Lch*Wpsa' AD='1.5*Lch*Wpsa' PS='2*1.5*Lch+Wpsa' PD='2*1.5*Lch+Wpsa'
Mp2 obSA obSA vdd   vdd Pch5 L=Lch W=Wpsa AS='1.5*Lch*Wpsa' AD='1.5*Lch*Wpsa' PS='2*1.5*Lch+Wpsa' PD='2*1.5*Lch+Wpsa'
Mn1 oSA  in1  int   0   Nch5 L=Lch W=Wnsa AS='1.5*Lch*Wnsa' AD='1.5*Lch*Wnsa' PS='2*1.5*Lch+Wnsa' PD='2*1.5*Lch+Wnsa'
Mn2 obSA in2  int   0   Nch5 L=Lch W=Wnsa AS='1.5*Lch*Wnsa' AD='1.5*Lch*Wnsa' PS='2*1.5*Lch+Wnsa' PD='2*1.5*Lch+Wnsa'
 
Mn3 int  vdd  0     0   Nch5 L=Lch W=Wnsa AS='1.5*Lch*Wnsa' AD='1.5*Lch*Wnsa' PS='2*1.5*Lch+Wnsa' PD='2*1.5*Lch+Wpsa'

* Output buffer
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Mn4 obInv oSA 0     0    Nch5 L=Lch W=Wnsa AS='1.5*Lch*Wnsa' AD='1.5*Lch*Wnsa' PS='2*1.5*Lch+Wnsa' PD='2*1.5*Lch+Wnsa'
Mp4 obInv oSA vdd   vdd  Pch5 L=Lch W=Wpsa AS='1.5*Lch*Wpsa' AD='1.5*Lch*Wpsa' PS='2*1.5*Lch+Wpsa' PD='2*1.5*Lch+Wpsa'
 
Mn5 o obInv   0     0    Nch5 L=Lch W=Wnsa AS='1.5*Lch*Wnsa' AD='1.5*Lch*Wnsa' PS='2*1.5*Lch+Wnsa' PD='2*1.5*Lch+Wnsa'
Mp5 o obInv   vdd   vdd  Pch5 L=Lch W=Wpsa AS='1.5*Lch*Wpsa' AD='1.5*Lch*Wpsa' PS='2*1.5*Lch+Wpsa' PD='2*1.5*Lch+Wpsa'
.ends
 
Last edited by a moderator:

You have not given any parametres, is your negative voltage -.01 or -200V?. If the negative voltage is not too high, just feed it via two high value resistors to a positive voltage and use the join between them as your comparison point. e.g. input = -.5V feed via a 1M and another 10 M to +5V would give the new input as 0V, feed via 4.5 M from +5V and the point becomes +.5V.
Frank
 
Last edited:

How about a schematic?
 

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