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# About PGA112 gain calculate confuse

Other Parts Discussed in Thread: PGA112

Hi Ti,

Im JD from Taiwan.

<1> Document issue

PGA112 datasheet ,page 36 table 9

I calculate the Maximum Signal by  Page 33   Vout = G x Vin -  AVdd /2 x(G -1).

I calculate the Max Signal and result is Vout =2 x 2.25 -3/2 x (2-1) = 3 V, Im confuse why document result is 2.95V.

<2>Design issue

Our company had design a small current detect ion current(1uA to 100uA)PCB.

Detect mode are 1uA and 100 uA.

We detect 100uA is no problem normally.

When We detect 1uA current the result are 0.98uA   normally.

But when we change other PGA112 to self test 1uA mode,the result are different ,maybe 1.00 or 1.01uA or 1.02 on the same PCB,some PGA112 is above the average(0.98).

I know the PGA112 gain tolerance is very small.

but this issue is going to make our energy map shift.

Thanks .

• Hello JD,

I will look into this for you.

Best,

Errol Leon
Precision Op Amp Applications
• Hello JD,

The answer to your first question is that there is an Output Swing Limit of 50mV to the supply rails for the PGA112 (refer to page 7 of datasheet).  I have attached this specification below.

Table 9 on page36, shows 2.95V because the supply voltage AVDD is 3V; which 2.95V is 50mV from the rail.

For the second question, would you be able to provide a schematic of this design to be able to eliminate variable?  This way I can understand the system and help provide the appropriate solution for this issue.

Best regards,

Errol Leon

Precision Op Amps Applications