[time-nuts] LPRO-101 with Brooks Shera's GPS locking circuit

[Dr Bruce Griffiths]

SAIDJACK at aol.com wrote:
> Hi Bruce,
>  
> great ideas, thanks!
>  
> Said
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>   
Said

Attached circuit illustrates how a PT100 may be interfaced to a single 
channel 24 bit sigma delta ADC.
For clarity details such as op amp power supply connections, supply 
bypass capacitors, opamp types and connection of unused parts of ICs 
have been omitted.
The intention is purely to illustrate a principle not to give a complete 
circuit diagram.

OPERATION:
Convert output
Reverse polarity
Convert output
Subtract second reading from first
etc.

To some extent all parts are temperature sensitive.
The subtraction  (ie correlated double sampling) of the 2 measurements 
ensures that offsets are relatively unimportant.

The resistor values may be adjusted to suit higher resistance 
temperature sensors: PT500, PT1000 etc.

Low on resistance power FETs are used to switch the polarity of the 
current flowing in the linearised bridge components.
This minimises the effect that the FET switch on resistance has on the 
gain tempco.
R105 is used purely to keep the reference current relatively independent 
of POLARITY.
Since it affects the gain slightly it would be better to use a separate 
switch to connect/disconnect one end from ground as required.

Bruce
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