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This figure shows an ADC.

My question: Why there is resistor (R) connecting to inverting pin of each OPAMP.

Are they really necessary? In my opinion, they can be removed because you will have to compare the voltage at non-inverting pin to the voltage at inverting pin anyway. And that is enough for converting analog Vin to digital signal.

So what's the point for having those resistors?

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    \$\begingroup\$ So where you would connect the inverting inputs? \$\endgroup\$ – G36 Oct 17 '20 at 17:06
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    \$\begingroup\$ If there are no resistors, all comparators compare with same voltage. Now each compator compares with different levels so the voltage can be measured how much is it. \$\endgroup\$ – Justme Oct 17 '20 at 17:07
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    \$\begingroup\$ The resistors divide a single reference voltage down to multiple reference voltages, so you can compare multiple voltage levels, forming an n-bit ADC. Without resistors, there's only a single Vref, and it becomes a 1-bit ADC (a.k.a a comparator) \$\endgroup\$ – 比尔盖子 Oct 17 '20 at 17:18
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    \$\begingroup\$ Please show “In my opinion, they can be removed because you will have to compare the voltage at non-inverting pin to the voltage at inverting pin anyway.” with a simulation and report back. \$\endgroup\$ – winny Oct 17 '20 at 17:38
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    \$\begingroup\$ The resistor ladder cannot be removed because it constitutes the entire idea of this particular form of analog-to-digital converters. And this is a direct parallel converter, not "successive approximation" one. \$\endgroup\$ – Ale..chenski Oct 18 '20 at 4:46
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In my opinion, they can be removed because you will have to compare the voltage at non-inverting pin to the voltage at inverting pin anyway.

You have to have a voltage to compare with.

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Figure 1. Apply 8 V to the Vref input and each of the comparators gets a proportional voltage.

In this case we've applied 8 V and there are seven steps so each comparator has a reference voltage between 1 and 7 volts.

So what's the point for having those resistors?

It's to generate the reference voltages.

  • Now if Vin < 1 V all comparators will be off.
  • When 1 V ≤ Vin < 2 V comparator 'a' will turn on.
  • When 2 V ≤ Vin < 3 V comparators 'a' and 'b' will turn on.
  • etc.
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The purpose of ADC is to transform a signal from the Analog (real) World to the Digital (coded) World so you need to tell the logic circuit what level the signal Vin has in each moment. The circuit you show is able to distinguish 8 levels: below Va, between Va and Vb, between Vb and Vc and so on until above Vg. For that purpose needs a voltage reference in each one to compare it with the input and that is achieved by the voltage dividir made by the resistor network, values are the following: Va = 1/8 of Vref; Vb = 2/8 of Vref; Vc = 3/8 of Vref; Vd = 4/8 of Vref; Ve = 5/8 of Vref; Vf = 6/8 of Vref; Vg = 7/8 of Vref

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  • \$\begingroup\$ I am confused. Do you mean that it is an 3-bit = 8 levels/values ADC?. But I have never of any ADC with 3-bit resolution/accuracy. \$\endgroup\$ – tlfong01 Oct 18 '20 at 10:11
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    \$\begingroup\$ Hello @tlfong01, yes, it is a 3-bit ADC, it comes from the 8 different values that are distinguished by the 7 voltage thresholds I mentioned above, made by the 8 resistors network. Indeed it is mentioned in the circuit drawing, its output says 3 digital output. Looks this is a conceptual ADC diagram for learning purposes, no much use in real world although might be some application for which is enough. \$\endgroup\$ – Eloy Calatrava Oct 18 '20 at 12:27
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    \$\begingroup\$ Ah I see. I agree that it is good for newbies for learning basic operational amplifier as comparators and the idea of ADC. Real life ADC like MCP3008 using successive approximation is actually very complicated - I remember it has a DAC to successively generate trial approximate analog values closer and closer to the to value compare, and finally get the results. Many thanks for the confirmation. Cheers. \$\endgroup\$ – tlfong01 Oct 18 '20 at 13:05

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