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AD8028 stability and DC errors vs Vicm

Question asked by mbart on Oct 9, 2014
Latest reply on Oct 14, 2014 by iamPhil

Hello,

 

1 - Stability :

After a stabilty analysis of AD8028 in follower mode, with no load, it appears that AD8028 is stable (phase margin >46°) if the feedback resistor is close to 0 ohms (<100ohms).

In my design, I first wished a feedback resistor equal to 649ohms, wich lead to  20° phase margin.

it is confirmed on page 19 of 24 : " The AD8027/AD8028 have an input capacitance of 2 pF. This input capacitance forms a pole with the amplifier’s feedback network, destabilizing the loop. For this reason, it is generally desirable to keep the source resistances below 500 Ω, unless some capacitance is included in the feedback network."

 

2 - Input bias current vs Vicm :

But if the feedback resistor is equal to 0ohms, i can't appear impedance of positive and negative inputs, wich lead to an offset error equal to :

Vos = Rs x Delta_Ibias_vs_Vicm = 649ohms x 12µA = 8mV

The value Delta_Ibias_vs_Vicm = 12µA = -8µA - 4µA is based on the figure 37, page 13 of 24, of the AD8028 datasheet.

I can't accept a such error, because it is dependant of the input common mode voltage, and create 8mV non-linearity...

Do I understood properly the datasheet ?

Do the input offset current may vary also with Vicm ?

 

3 - Input offset voltage vs Vicm :

Even the input offset voltage may vary to 60µV with Vicm (cf Figure 41 page 14 of 24), wich lead to a non-linearity of 60µV ?

 

4 - Crossover region :

Most of these errors are caused by the crossover region. The best solution should be to power supply AD8028 between +5V and -2V, with input voltage between 0V and 4V, and to force SELECT = HIGH ?

Could you tell me the voltage to apply to the SELECT pin in the following power supplies configuration : -Vs = -2V et +Vs = +5V?

How is wired the SELECT pin for AD8028 in SOIC-8 ? SELECT = TRI or HIGH ?

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