Problem
I am having problems with the 2nd question and the 5th question
Q2) Analyze the errors due to using a TLC277CP opamp
AND
Q5) What is the maximum number of bits (for another DAC having the same current O/P
range) which can be accurately used if the circuit is conditioned using an OPA177GP?
(Neglect sources of error other than the opamp)
Attempt at a solution
The general formula for finding the DC imperfection for op amp circuits would be Verror(RTO) = Ib*Rf + Vos*Gain. (and ignore Ios as a shortcut because it is insignificant in comparison to Ib error) But in this case, how do you find the gain when there is only one resistor in the circuit?
For finding the ENOB using the error from the op amp, my lecturer showed us this equation Vfs/2^N > Vos + Ib*Rf
where Vfs is full scale voltage, N is number of bits and right hand side is the DC error due to op amp. After substituting the Vos and Ib values from the datasheet into the eqn, i cannot get the answer she came up with - 15.xx
Here are my workings:
1/2^N > (60*10^-6) + (2800*10^-8)
1/(60*10^-6) + (2800*10^-8) > 2^N
log (1/(60*10^-6) + (2800*10^-8)) > log2^N
log (1/(60*10^-6) + (2800*10^-8)) > Nlog2
log (1/(60*10^-6) + (2800*10^-8)) / log2 > N
13.47(bits) > N
So i tried to find using algebra what was the missing variable that i did not account and derived this.
Verror t < (60*10^-6) + (2800*10^-8)
So the Verror is too big. It needs to be smaller to get a higher value of N.
Using a different datasheet, i used the typical Vos and Ib errors instead of the max Vos and Ib errors and managed to get the values. But i arrived at this answer purely using maths. Could anyone explain why this so to me?
I am not sure what i am missing and would appreciate any hints towards getting there..
I am having problems with the 2nd question and the 5th question
Q2) Analyze the errors due to using a TLC277CP opamp
AND
Q5) What is the maximum number of bits (for another DAC having the same current O/P
range) which can be accurately used if the circuit is conditioned using an OPA177GP?
(Neglect sources of error other than the opamp)
Attempt at a solution
The general formula for finding the DC imperfection for op amp circuits would be Verror(RTO) = Ib*Rf + Vos*Gain. (and ignore Ios as a shortcut because it is insignificant in comparison to Ib error) But in this case, how do you find the gain when there is only one resistor in the circuit?
For finding the ENOB using the error from the op amp, my lecturer showed us this equation Vfs/2^N > Vos + Ib*Rf
where Vfs is full scale voltage, N is number of bits and right hand side is the DC error due to op amp. After substituting the Vos and Ib values from the datasheet into the eqn, i cannot get the answer she came up with - 15.xx
Here are my workings:
1/2^N > (60*10^-6) + (2800*10^-8)
1/(60*10^-6) + (2800*10^-8) > 2^N
log (1/(60*10^-6) + (2800*10^-8)) > log2^N
log (1/(60*10^-6) + (2800*10^-8)) > Nlog2
log (1/(60*10^-6) + (2800*10^-8)) / log2 > N
13.47(bits) > N
So i tried to find using algebra what was the missing variable that i did not account and derived this.
Verror t < (60*10^-6) + (2800*10^-8)
So the Verror is too big. It needs to be smaller to get a higher value of N.
Using a different datasheet, i used the typical Vos and Ib errors instead of the max Vos and Ib errors and managed to get the values. But i arrived at this answer purely using maths. Could anyone explain why this so to me?
I am not sure what i am missing and would appreciate any hints towards getting there..
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