[sdiy] Help, I'm Desperate! (Charge Injection with DG408)

[Roman Sowa]

Why not simply LM3914 and 4066?

Roman

W dniu 2018-12-05 o 13:05, Steve Lenham pisze:
> On 05/12/2018 06:55, David G Dixon wrote:
> 
>> Today, I thought deeply about the switching glitch problem, and I think I
>> concur with Steve Lenham.
> 
> Go me! ;-)
> 
> <snip>
>>
>> So, I now believe that the best approach is to use Gray code to drive the
>> switches, so that only one bit ever has to change.  This consists of two
>> things: 1) changing the logic circuit to generate Gray code from the CV
>> signal, and 2) reconnecting the switch inputs to conform to Gray 
>> code.  The
>> second part is trivially easy.  The first part is not so easy.
>>
>> However, I have discovered that 3-bit binary can be converted to 3-bit 
>> Gray
>> code fairly directly, since the 1st Gray and binary bits are 
>> equivalent, the
>> 2nd Gray bit is the XOR of the 1st and 2nd binary bits, and the 3rd 
>> Gray bit
>> is the XOR of the 2nd and 3rd binary bits.  An analog XOR circuit like 
>> the
>> one on Ian Fritz's website (inspired by Electronotes) would fulfill 
>> the XOR
>> function from my existing 3-bit hack-job ADC.
> 
> Gray code will not help you unless you can generate it directly.
> 
> Taking a binary address that passes through multiple transitional states 
> and converting it to Gray code will give you a Gray code sequence that 
> does exactly the same.
> 
> If your binary address is supposed to be changing from 1 to 2, but also 
> temporarily passes through 3, then passing this value through a Gray 
> encoder will simply give you an address that goes from the Gray code for 
> 1 (001) to the Gray code for 2 (011) but also temporarily passing 
> through the Gray code for 3 (010). Nothing will have been gained.
> 
> Also, Gray code is only beneficial when the encoded value is guaranteed 
> to only increment or decrement by one unit at a time (for example in 
> rotary encoders). Otherwise, the statement that "only one bit of a Gray 
> code value changes at a time" does not hold!
> 
> The Gray code values corresponding to decimal 0-7 are:
> 
> 000, 001, 011, 010, 110, 111, 101, 100
> 
> Clearly a change in the value greater than one often involves changing 
> more than one bit of the Gray code.
> 
>> However, I am going to rethink this entire logic and switching circuit,
>> because it is getting fairly complicated.  I believe that proper 
>> comparators
>> with pullup resistors to 5V would be better than the opamps with zener
>> diodes, and I'll figure out some other way to generate the threshold
>> voltages.  With proper logic levels, I'll probably just use proper logic
>> chips.
> 
> I can't help agreeing with Tom in that one of the 8-pin PICs with an 
> onboard ADC would kill the problem stone dead very inexpensively, but I 
> appreciate that not everyone has the facilities/experience/inclination 
> to take that approach.
> 
> As an alternative I would offer the following:
> 
> - Generate a clock signal at a fairly high frequency.
> - Use the rising edge of that clock to trigger a sample-and-hold circuit 
> (could be just a CMOS analogue switch).
> - Feed the sampled signal to your comparators.
> - Take the comparator outputs to a D-type latch.
> - Clock the latch with the falling edge of your sample clock.
> - Control your analogue multiplexer with the outputs of the latch.
> 
> That is effectively what the PIC would be doing, just in discrete form. 
> The comparators then have half the clock period to settle to their final 
> values.
> 
> Good luck!
> 
> Steve L.
> Benden Sound Technology
> 
> 
> 
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