- #1

jf623

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I'm trying this problem on allaboutcircuits too. However they havn't been as helpful as this board is for other problems (mostly mathematics).

I have been given an assignment which is part of, what I consider, the most difficult part of electronics engineering. Circuit analysis. It is to analyse the attached circuit, which is a fuel level meter for a car. It is a design for an ADC to convert an analogue voltage (v_fuel) ranging from 0 to 3V (3V being a full tank) into a 4 bit binary number, which is then sent to a 7 segment display.

(a) Calculate suitable resistances for R1 to R5 so that the binary values from the counter produce voltages from 0 V to approximately 3.75 V in approximately 0.25 V steps at point K in the circuit.

(b) Explain and sketch graphs of what is happening at points J, K, L.

Resistors R1 to R4 are related in that R3=2*R4, R2=2*R3, R1=2*R2.

OA1, OA2, OA3 are all HCT logic family op-amps (TL074).

Vp = 5V, Vm = -5V.

I understand that the resistor network, and, OA1, OA2, OA3 are all together a DAC. OA2 is simply inverting and isolating, OA3 is definitely a comparator and I understand that more than any other part. I know OA1 is a summing amplifier, but I am uncertain how it relates to the resistor network. I think that it is shifting the voltage range from 0V -> 5V, to 0V -> 3.75V. I am having extreme difficulty approaching this problem, I have all the lecture notes and books on this topic. Should I treat the summing amplifier as an inverting negative feedback op-amp, and find a fractional gain of it, then apply the descending values of resistance? (b) should be straightforward once I understand (a).

## Homework Statement

I have been given an assignment which is part of, what I consider, the most difficult part of electronics engineering. Circuit analysis. It is to analyse the attached circuit, which is a fuel level meter for a car. It is a design for an ADC to convert an analogue voltage (v_fuel) ranging from 0 to 3V (3V being a full tank) into a 4 bit binary number, which is then sent to a 7 segment display.

(a) Calculate suitable resistances for R1 to R5 so that the binary values from the counter produce voltages from 0 V to approximately 3.75 V in approximately 0.25 V steps at point K in the circuit.

(b) Explain and sketch graphs of what is happening at points J, K, L.

## Homework Equations

Resistors R1 to R4 are related in that R3=2*R4, R2=2*R3, R1=2*R2.

OA1, OA2, OA3 are all HCT logic family op-amps (TL074).

Vp = 5V, Vm = -5V.

## The Attempt at a Solution

I understand that the resistor network, and, OA1, OA2, OA3 are all together a DAC. OA2 is simply inverting and isolating, OA3 is definitely a comparator and I understand that more than any other part. I know OA1 is a summing amplifier, but I am uncertain how it relates to the resistor network. I think that it is shifting the voltage range from 0V -> 5V, to 0V -> 3.75V. I am having extreme difficulty approaching this problem, I have all the lecture notes and books on this topic. Should I treat the summing amplifier as an inverting negative feedback op-amp, and find a fractional gain of it, then apply the descending values of resistance? (b) should be straightforward once I understand (a).