DAC circuit - Page 1
Forum section: electronics / beginners

author: nForce
author_rank: Frequent Contributor
timestamp: January 24, 2019, 06:38:56 pm
content: How do we get "K" in this circuit? Is the calculation correct?Thanks.

author: awallin
author_rank: Frequent Contributor
timestamp: January 24, 2019, 06:52:53 pm
content: didn't your teacher teach you to use units after each number?it would also help if you'd specify the unit you want for "K"

author: nForce
author_rank: Frequent Contributor
timestamp: January 24, 2019, 06:58:35 pm
content: I think here we don't have any units. K is just a constant to get from ADC to the motor.

author: Benta
author_rank: Super Contributor
timestamp: January 24, 2019, 07:29:39 pm
content: Calculation looks OK. Unit is RPM/bit.

author: nForce
author_rank: Frequent Contributor
timestamp: January 24, 2019, 08:04:37 pm
content: Quote from: Benta on January 24, 2019, 07:29:39 pmOk Benta, if you understand, can you explain how do I get "K" and some comments about derivation?

author: Wimberleytech
author_rank: Super Contributor
timestamp: January 24, 2019, 11:19:48 pm
content: Quote from: nForce on January 24, 2019, 06:38:56 pm

author: nForce
author_rank: Frequent Contributor
timestamp: January 25, 2019, 06:04:42 pm
content: Can anyone explain to me how do we get "K"?

author: Wimberleytech
author_rank: Super Contributor
timestamp: January 25, 2019, 06:39:16 pm
content: Quote from: nForce on January 25, 2019, 06:04:42 pmMy earlier comment was wrong.

author: nForce
author_rank: Frequent Contributor
timestamp: January 25, 2019, 07:16:57 pm
content: This is great, but can you explain/comment in words what are we doing here? Some description.

author: Wimberleytech
author_rank: Super Contributor
timestamp: January 25, 2019, 07:29:30 pm
content: Quote from: nForce on January 25, 2019, 07:16:57 pmK is the small-signal gain from the digital input to the RPM of the motor.If D steps from 0 to 4095, a delta of 4095, then:4095 * (3.3/4095) (10/1.65) (1500/10) = 3000. The output rpm changes from -1500 to +1500...a total output delta of 3000.For a 1 LSB delta for D, the rpm changes:1 * (3.3/4095) (10/1.65) (1500/10) = 0.7326 rpm

author: nForce
author_rank: Frequent Contributor
timestamp: January 26, 2019, 12:08:35 pm
content: Sorry, but I still don't understand. What do you mean by D? DAC? And what is delta?So DAC outputs a voltage between 0 and 3.3 V. And because it's 12-bit DAC the resolution is 3.3/4095?What about those op amps? And there is +10 V or -10 V, shouldn't be the range then 20 V?

author: Wimberleytech
author_rank: Super Contributor
timestamp: January 26, 2019, 01:20:25 pm
content: Quote from: nForce on January 26, 2019, 12:08:35 pmD is the digital input. It has a value between 0 and 4095. When D=0, the output of the DAC is 0, when D=4095, the output of the DAC is 3.3."delta" is a change. 0 to 3.3V is a delta of 3.3V. 1V to 2V is a delta of 1V.QuoteThe size of an LSB is 3.3/4095. The smallest unit it can resolve is 1 LSB.QuoteThe opamps do what opamps do.Yes, -10V to +10V is a delta of 20V

author: Wimberleytech
author_rank: Super Contributor
timestamp: January 26, 2019, 01:39:38 pm
content: Here is the circuit with some example values.Use your circuit analysis skills and analyze the signal path.

author: nForce
author_rank: Frequent Contributor
timestamp: January 26, 2019, 02:30:00 pm
content: So V1 = 1.65 V.And your U1 op amp has a gain of 3. In my case since the resistors are all the same, it has a gain of 2.So: 1.65 * 2 = 3.3, and because the delta is 3.3 then 3.3/3.3 = 1 which is A = 1.Right? But why is delta important here?Thank you for your effort.

author: Wimberleytech
author_rank: Super Contributor
timestamp: January 26, 2019, 02:38:11 pm
content: Delta is important because that is what the original "K" equation was calculating. Calculating small signal gain. Dont worry about the -1.65 V voltage source when calculating K.

author: nForce
author_rank: Frequent Contributor
timestamp: January 26, 2019, 04:05:27 pm
content: Thanks. So the second term is 10/1.65, because on the output we have 10 V, and at the input, it's 1.65 V. So the gain is 10/1.65, but why 10, because delta is 20V? And why at the motor it's 1500/10, here we are mixing rpm and voltage.

author: Wimberleytech
author_rank: Super Contributor
timestamp: January 26, 2019, 06:26:33 pm
content: Quote from: nForce on January 26, 2019, 04:05:27 pmQuoteThe motor speed is 150 rpm/volt. 10 volts yields 1500 rpm. Ergo..."10V"

author: nForce
author_rank: Frequent Contributor
timestamp: January 26, 2019, 07:44:12 pm
content: Yes, but delta, as you said why is important: Delta at the motor, is 3000 rpm. Not only 1500 rpm.

author: Wimberleytech
author_rank: Super Contributor
timestamp: January 26, 2019, 08:00:27 pm
content: "K" is the gain of the system: K = Speed/D. D is the digital input to the DAC.K is important because this is likely part of a control system where the motor speed is sensed and fed back to the input of the DAC to maintain desired speed.So, it is a control system (I assume). In a control system, K is an important parameter associated with, among other things, stability.If what I just told you makes no sense, then there is no value in me explaining "delta."

author: nForce
author_rank: Frequent Contributor
timestamp: January 26, 2019, 08:10:58 pm
content: Yes, I understand you.I am just using the knowledge from known values:First-term the numerator has delta which is 3.3 V so the same reason for the motor the numerator would have 3000 which is delta. Delta is the difference between min and max.

author: Wimberleytech
author_rank: Super Contributor
timestamp: January 26, 2019, 09:05:46 pm
content: Quote from: nForce on January 26, 2019, 08:10:58 pmNo, the first expression converts bits to volts. The second expression converts volts to volts. The third expression converts volts to RPM.In term of units:(Volts/bit) (Volts/Volts) (RPM/Volts) = RPM/bit <--KIf your DAC input changed from 2000 to 2001, a delta of 1 bit, then using the equation, RPM would change by a delta of:1 * (3.3/4095) * (10/1.65) * (1500/10) = 0.732 RPM

author: nForce
author_rank: Frequent Contributor
timestamp: January 26, 2019, 10:14:49 pm
content: OK, now I understand in terms of units.Can you just say why is then 1500 rpm? Or maybe another person can write it.Let's be consistent:The first term: delta = 3.3 V - 0 V = 3.3 VLast term: delta = 1500 rpm - -1500rpm = 3000 rpm.

author: Wimberleytech
author_rank: Super Contributor
timestamp: January 26, 2019, 10:27:55 pm
content: Quote from: nForce on January 26, 2019, 10:14:49 pmNo, you misunderstand "delta"In the first term 3.3/4095 just the transfer function from the D input to the output of the first amplifier.As far as 1500rpm. That is just the designer's choice based on motor used. No magic.

author: nForce
author_rank: Frequent Contributor
timestamp: January 26, 2019, 10:42:39 pm
content: Why the first term the range 0-3.3 V is not delta? It seems to me that all the data on the schematic is consistent.Omg, this is going to take forever.

author: Wimberleytech
author_rank: Super Contributor
timestamp: January 26, 2019, 10:53:34 pm
content: Quote from: nForce on January 26, 2019, 10:42:39 pm3.3V - 0V is indeed a "delta"BUT, that is not the first term. The first term is a transfer function as I said.How long this takes depends on you. I understand the circuit and the equations.You should install LTSPice if you have not already done so. I will upload the LTSPice file and you can play with it. Maybe it will help you get a feel for what is going on.

author: nForce
author_rank: Frequent Contributor
timestamp: January 27, 2019, 10:09:01 am
content: What is here a transfer function? You said that 3.3 V is delta, why don't we use at the motor delta = 3000 rpm?

author: nForce
author_rank: Frequent Contributor
timestamp: January 27, 2019, 02:25:42 pm
content: Sorry, but only one person on this forum understands the circuit?