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Aug 1st, 2021, 10:30pm
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 Passive adder in continuous time delta sigma modulator (Read 736 times)
 polyam Community Member Offline Posts: 71 Passive adder in continuous time delta sigma modulator May 05th, 2017, 10:29am   Hi,I am using a feedforward (FF) topology to implement my CT delta sigma modulator (CT-DSM). To implement the adder before the quantizer, I want to use a passive adder instead of an active one. In doing so, I've done a hand calculation (please see the attachment). You'll see 3 FF coefficients (k1, k2, k3) and a feedback DAC. I am using a current steering DAC.To realize those coefficients, I am using three resistors (R1, R2, R3). A simple paper and pencil calculation was done and R1,2,3 were chosen. I'm a little bit puzzled because when I plug in the selected R1,2,3 to the relations 1 2 3 (in the attachment) I don't get the correct coefficients. (please see (5) (6) and (7)). If I multiply (5) (6) and (7) in a constant value of 6.5 dreams come true !!! I don't know if I've really done a right job or not !!Any idea to resolve this problem would be appreciated.tnx Back to top IP Logged
 cheap_salary Senior Member Offline Posts: 162 Re: Passive adder in continuous time delta sigma modulator Reply #1 - May 6th, 2017, 6:32am   Can you surely understand |k1|+|k2|+|k3|<1.0 for passive summation ? Back to top IP Logged
 polyam Community Member Offline Posts: 71 Re: Passive adder in continuous time delta sigma modulator Reply #2 - May 6th, 2017, 8:11am   Hi cheap_salary,Honestly, I guessed that but I was not sure! So, you would say there is no way to implement a passive adder for such a noise transfer function (1-z^-1)^2? and the active one is the only solution?What if I reduced Hinf to make the NTF appropriate for passive one?tnx Back to top « Last Edit: May 6th, 2017, 7:23pm by polyam »     IP Logged
 cheap_salary Senior Member Offline Posts: 162 Re: Passive adder in continuous time delta sigma modulator Reply #3 - May 6th, 2017, 9:56pm   polyam wrote on May 6th, 2017, 8:11am:So, you would say there is no way to implement a passive adder for such a noise transfer function (1-z^-1)^2?and the active one is the only solution? No.Surely understand passive summation.How can you realze "K1=1.0, K2=2.5, K3=3.0" by R1, R2 and R3 ?|K1|+|K2|+|K3|+|Kdac| > 1.0 is never possible.Of course, you can not realize "K1=1.0, K2=2.5, K3=3.0, Kdac=-2.0".K1=1.0, K2=2.5, K3=3.0, Kdac=-2.0|K1|+|K2|+|K3|+|Kdac|=8.5k1=K1/8.5=1.0/8.5k2=K2/8.5=2.5/8.5k3=K3/8.5=3.0/8.5kdac=Kdac/8.5=-2.0/8.5You have to compensate scale down of 1.0/8.5 by decreasing LSB of quantizer.If your quantizer is a 1bit, compensation is not needed. Back to top IP Logged
 polyam Community Member Offline Posts: 71 Re: Passive adder in continuous time delta sigma modulator Reply #4 - May 10th, 2017, 9:29am   Thank you Cheap_salary,It makes sense. you've made it clear to me.One more question! Let's say the modulator is completely idealized. Should we expect a very severe performance degradation (noise shaping form, SQNR) when a passive adder is used? (comparing to let's say active adder)?Tnx Back to top IP Logged
 cheap_salary Senior Member Offline Posts: 162 Re: Passive adder in continuous time delta sigma modulator Reply #5 - May 11th, 2017, 4:46am   polyam wrote on May 10th, 2017, 9:29am:Let's say the modulator is completely idealized.Should we expect a very severe performance degradation (noise shaping form, SQNR) when a passive adder is used? No, as far as internal ADC could be enough accurate.In addition to accuracy of internal ADC, there is an issue of accuracy for passive summation itself.This is true for Switched-Capacitor-DSM.Passive summation is comprised from capacitors.Accuracy of summation could be affected by parasitic capacitor.Output signal of passive summation is very small, so it could be degraded by parasitic.This is true for CT-DSM where passive summation is comprised from resistors.However there are some merits for passive summation.(1) No distortion(2) High Speed(3) Ultra low powerThese are difficult to realize in active summation. Back to top IP Logged
 polyam Community Member Offline Posts: 71 Re: Passive adder in continuous time delta sigma modulator Reply #6 - May 22nd, 2017, 7:20am   Thank you cheap-salary. The information you gave me was helpful. Back to top IP Logged
 Frank_Heart Junior Member Offline Posts: 29 Re: Passive adder in continuous time delta sigma modulator Reply #7 - Nov 16th, 2017, 10:16pm   Inspiring discussion!So the output of passive summer is pretty small, which could make the quantizer design much more challenging?The input of quantizer is output + shaped-noise, which is equal to input + shaped-noise, does this mean Max input range will be limited by passive adder？ If so, shall we increase the FBDAC gain? Back to top IP Logged
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