[volt-nuts] 32-Bit PWM divider

[Bruce Griffiths]

Andreas
>> 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.
>>
>>      
Incorrect, the output of the error integrator is (or should be) sampled 
when its output is small except during the first cycle after a large 
step change in the MS PWM duty cycle.
Thus the current transients in the sampling capacitor and switch should 
be relatively small once the circuit has settled.

>> 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.
>>
>>      
If the difference in output corresponding to the 2 PWM values is small 
as is almost always the case when the DAC is used to set the EFC voltage 
of the OCXO in a GPSDO or similar control loop such behaviour shouldn't 
be an issue.

>> 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.
>>
>>      
Dielectric absorption will increase the settling time but shouldnt 
produce staircase effects.
However, Mylar has significant 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
>>
>>      
Bruce