4 Replies Latest reply on Jul 6, 2010 9:42 AM by userc_40597

# delta sigma instrument. amp

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);?

• ###### 1. Re: delta sigma instrument. amp

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?

• ###### 2. Re: delta sigma instrument. amp

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

• ###### 3. Re: delta sigma instrument. amp

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

I guess ADC Delsig gives always 2's complement outputs

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

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)

• ###### 4. Re: delta sigma instrument. amp

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