delta sigma instrument. amp

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Anonymous
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I'm trying to use del sig adc plus 2 pga's as an instrumentation amplifier.(acc to AN60319)

   

ADC datasheet says, GetResult16() returns signed value; GetResult32() (not known?!!!! - I presume unsigned)

   

In single input ADC mode, the -Input should be tied to VSS. How it is in differetial mode?

   

ADC normally cannot measure negative values.

   

What is the input range (PGA gain 1; ADC gain 1,differential input,internall vref=1.024V)  for PGA+ and PGA- pins?

   

Example:

   

PGA+ = 0V; PGA - = 1mV or PGA+=1mV, PGA-=0V,diff mode,vref=1.024V what will be the output counts(res 20bits);?

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4 Replies
Anonymous
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In other words:

   

Counts = Vd*gain/Vref * 2^(n-1) (an60319 page 2) Vd-differetial voltage= Vpositive - Vnegative, n-resolution

   

What counts will be if Vd < 0,Vd = 0,Vd>0?

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Anonymous
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 Hi Robert,

   

The ADC by itself can provide a gain of 32 in the component now. It can provide a maximum gain of 128 in the future version(Beta 5) of PSoC creator. So, just using the ADC will be sufficient for gains less than 128. For gains greater than 128, please refer to the connection shown in this KB

   

http://www.cypress.com/?id=4&rID=40623

   

Let me know if your design requires more gain or some other necessity prevents the use of this design

   

Regards,
Praveen

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Anonymous
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It's not the issue of gain but the ADC output.

   

I guess ADC Delsig gives always 2's complement outputs

   

ADC_input_plus = 0V

   

ADC_input_minus = 1mV

   

then if Vref=1.024V,gain 1,res=20bit

   

(((0V-1mV)*1)/(1.024+1mV))* 2^(20-1)=511 or 523777

   

    Vdiff       K       Inp Range    Max counts

   

Now if

   

ADC_input_plus = 999mV

   

ADC_input_minus = 1V

   

then if Vref=1.024V,gain 1,res=20bit

   

(((999mV-1000mV)*1)/(2000mV))* 2^(20-1)=262 or 524026

   

             Vdiff          K       Inp Range    Max counts

   

 

   

Why same difference give different results?

   

Because measurement range changes with -Input DC value   (acc. to DelSig manual)

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Anonymous
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2000mV should be replaced with 2024mV to be honest (-Input +- Vref)

   

In other words:

   

Is there a method to measure voltages around 0 like +- 2mV?

   

regards

   

robert

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