[sdiy] CEM3374 vco temp sensor

[Scott Bernardi]

Are you sure that's true for the CEM3374, Paul? I know it's true for the 
3340. The 3374 datasheet says "...the 3374 includes an on-chip 
temperature sensor which generates an output voltage, nominally +2.5v, 
proportional to chip temperature (i.e. with a TC of +3300 ppm); by 
ensuring that the exponential control voltage is derived from this 
sensor output (for instance, by applying it to the reference input of 
the system DAC), unsurpassed oscillator stability can be acheived).
The sensor voltage mentioned most certainly will be a band gap generated 
voltage. This can be scaled internally to +3300ppm by a simple linear 
amplifier and does not require a multiplier. The multiplication is done 
by applying it to the reference voltage of a multiplying DAC whose 
digital inputs are the control voltage.  Once upon a time I looked into 
using AD633's to act as the external multipliers, but ended up using a 
couple of tempco resistors instead 
(http://home.comcast.net/~sbernardi/elec/og2/og2_cem3374.html).  I think 
Jim's idea of using the LM13700 multiplier like in his temp compensated 
VCO would work fine. You'd need two of them.
The 3340 on the other hand, has an onboard analog multiplier which 
multiplies the internal PTAT voltage times the input control voltage. 


Paul Schreiber wrote:

>Correct, it uses an on-chip multiplier.
>
>Paul S.
>
>
>  
>
    Karl Dalen wrote:

    It also say that a ±90mv covers a 10 octave range
    wouldent that put quite some precision requirements
    on the OP amps in the DAC reference feedback path
    in terms of V offset drift with TC?!

The DAC should run at the full scale voltages (1v/octave).  Scale it 
down after the DAC with an inverter, 100K input R and a say a 1.5K + 500 
trimmer (for scale trim) in the feedback path. . This is no different 
than what other VCO designs have.


Its no doubt that it works splendid but how sure
could one be that the temp drift are equal on both
VCO's?

  

There should be pretty good matching between the two VCO's with their 
exponential transistors and the bandgap generator transistors close to 
each other on the same chip.

-- 

Scott Bernardi
sbernardi at comcast.net

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20031017/d119238a/attachment.htm>