[volt-nuts] 32-Bit PWM divider

[Ulrich Bangert]

Hello Andreas,

when I came over this circuit I was in search for a 24 Bit DAC for the EFC
of an OCXO. For hat reason everything was a bit less ambitious than you are
in search for. The reference I used was a LT1021, much inferior than the
LTZ1000 in terms of tempco but no problem for this application because its
use inside a closed regulation loop. The resistor networks were computed
new, so i have no experience with it's original values. 

The noise measurements that I made were done with a HP3457 in
"high-resolution" mode (7-1/2 digits). The HP3457 was set to integrate over
50 power line cycles. If I remember correct the noise that i measured was
pretty much that of the reference divided down according to the pwm ratio.  

Best regards
Ulrich Bangert

> -----Ursprungliche Nachricht-----
> Von: volt-nuts-bounces at febo.com 
> [mailto:volt-nuts-bounces at febo.com] Im Auftrag von Andreas Jahn
> Gesendet: Sonntag, 3. Oktober 2010 23:11
> An: Discussion of precise voltage measurement
> Betreff: [volt-nuts] 32-Bit PWM divider
> 
> 
> On April, 24th Ulrich Bangert wrote:
> 
> >while I have no experience at all with the AD5791 I would 
> like to draw 
> >your attention to the fact that even higher reolution monotonic DACs 
> >are easily constructed from "normal" microcontroller's PWM outputs. 
> >Well, of course you need some additional electronics like in
> 
> >http://www.electronicsweekly.com/Articles/2008/10/30/44817/dc
> -accurate-
> >32-bit-dac-achieves-32-bit-resolution.htm
> 
> >but this stuff really works!
> 
> I have built the cirquit from EDN (with some modifications) 
> which seems to 
> be the same that Ulrich used: 
> http://www.edn.com/article/471981-DC_accurate_32_bit_DAC_achie
> ves_32_bit_resolution.php
> 
> but the values that are stated in the article are far beyond 
> that what I 
> have measured.
> 
> My modifications are: using MAX6250A instead of AD586L.
> R2,R3 = 51R and R6 = 0R so that I get around 30 Bits 
> resolution with 2 Bits 
> overlap.
> C1 = 100nF , R1 = 100K with 100 Hz period. (I tried 244 Hz 
> period with R1 = 
> 41K but this has nearly no impact)
> All built on a soldered breadboard using star grounding.
> 
> The issues that I have with the cirquit are:
> 
> 1. the given formula for output voltage is not correct. When 
> recalculating I 
> get 50% from VRef at PWMH = 8000 and PWML = 8000 since the 
> voltages are not 
> added. They are mixed in relation to the resistor values. The 
> maximum output 
> voltage one can get is 65535/65536 * VRef.
> 
> 2. Linearity: With the original 5 Ohms Resistor I measured 
> around 60-65uV 
> maximum linearity deviation at 2.5V Output. This would give 
> about 16 Bits 
> Linearity in my case. Carefully adjusting R7 to linearity 
> (7.18 Ohms) gives 
> around 12-15uV (3ppm) linearity. But this is far from the 22 
> bits = 0.3 ppm 
> linearity stated in the article. The linearity curve is 
> relative flat up to 
> 75-80% of the range. Above 80% the linearity error increases 
> continously.
> 
> 3. settling time: Although having carefully adapted the 
> integration time 
> constant to the period time the output gets only 80-90% of 
> the voltage step 
> in the first step. When looking at the oscilloscope the 
> integrator output 
> breakes down at the time when S3 switches. So it needs around 
> 5 PWM cycles 
> to settle on the scope.
> 
> 4. Large current spikes through S3: At 2.5V Output the 
> integration capacitor 
> will have 2.5Vpp Integration ripple. When having a 50% duty 
> cycle for S3 
> there will be current spikes of 2.5V limited only by the 200R 
> resistor and 
> the resistance of the switch. These spikes increase the 
> output noise. (I 
> measured around 100uVpp with a LTC2400 A/D-Converter). 
> Increasing the PWM 
> duty cycle to around 97% will give a good compromize between 
> noise and 
> settling time of the output capacitors.
> 
> 5. large negative output spikes on voltage steps >= around 0.5V: When 
> switching between 2 PWM duty cycle values I see large 
> negative spikes (up to 
> negative voltages) on the output voltage. For me it seems 
> that the input 
> protection diodes between negative and positive input (1.1V 
> forward voltage) 
> are going to be forward biased.
> 
> 6. Output noise measured with a bandwidth limited fourth 
> order (0.1Hz to 10 
> Hz) amplifier (10000 fold) shows 10-15uVpp "noise". This noise is not 
> randomly distributed but shows some cyclical staircased 
> behaviour. The 100nF capacitor for the first integrator is a 
> polypropylene type. The 
> two 1uF capacitors are still mylar type. Perhaps the "staircase" has 
> something to do with dielectric absorption.
> 
> Has anybody any idea how to get better results from this 
> cirquit? Anyone 
> else who built this cirquit?
> 
> @Ulrich: how did you get your good results from noise 
> measuring (only the 
> LTZ1021 noise = 3uVpp)
>     Which capacitor types did you use? mylar or polypropylene or else?
>      Any hints on the building of the cirquit?
> 
> best regards
> 
> Andreas
> 
> 
> 
> 
> _______________________________________________
> volt-nuts mailing list -- volt-nuts at febo.com
> To unsubscribe, go to 
> https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
> and follow the instructions there.