[hpsdr] Odyssey Siren Design

[Joseph Julicher]

I was concerned about the output swing.
Thanks for recommending a better amplifier.


On Sep 19, 2006, at 10:31 PM, Lyle Johnson wrote:

>> I am about 80% finished in the schematic entry on Siren and ran  
>> into an interesting design idea that may help simplify the power  
>> supply for use in portable (or extraterrestrial) designs.
>> We are using the TI TLV320AIC31 Codec which has differential  
>> inputs and outputs.
>> The I/Q demodulator is the standard QSD circuit.
>> To improve the S/N ratio a good instrumentation amplifier is  
>> typically used after the QSD commutator and before the CODEC.
>> However, there are two problems:
>> 1) the INAMP requires a bipolar power supply, and
>> 2) the TI CODEC has a differential input and the INAMP will  
>> convert the differential QSD signal to a single ended signal.
>> I am proposing the following solution:
>> Put a low noise 10x buffer on each line beween the codec and the qsd.
>> The LMH6624 is a low noise opamp from National.  At 1MHz it has  
>> 0.92nv/sqrt(hz) noise.  That is quite low.
>> http://www.national.com/pf/LM/LMH6624.html
>> The + and - signals are directly fed to the CODEC.  The circuit  
>> shown provides the following transfer function.
>> Vout = (Vin - 1.65)10 + 1.65
>
> It will only have an output swing of 2V (+/- 1V about the center)  
> with a single 5V supply, and each amplifier consumes 12 to 18 mA.
>
> A Linear Tech LT6231 is 1.1 nV-rt-Hz (at 10 kHz), stable at unity  
> gain, draws 3.5 mA/amplifier, rail-to-rail output.
>
> Anyway, I like the idea of a low-noise buffer to reduce the  
> equivalent noise figure of the ADC input!
>
> 73,
>
> Lyle KK7P
>


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