Dear all,

I probably posted a message in the wrong forum. Please find below the question related to ADA4084

Datasheet Table 3 specifies that at +/-5V, typical input impedance is [100KOhms||1.1pF]Differential and [200MOhms||2.5pF]Common mode.

Are these parameters guaranteed by design and can they be considered stable from one production batch to another?

In other words, for a voltage follower circuit, if we want to have a first order passive filter, is it possible to only add a resistor in series on the non inverting input? Can we considered the following circuits as equivalent?

In addition to this, regarding Isy parameter specifies at 0.595mA. Does it mean that total opamp supply current is 0.595mA? Or, does it mean 0.595mA on +5V rail and 0.595mA on -5V rail implying total current of 2x0.595=1,19mA?

Best regards,

Benoît,

Parents
• Benoit,

The 100k is calculated for a bipolar diff pair.  The capacitances are measured.

For your second schematic, you have the 2.5 pF common mode, but you also have 1.1 pF to AC ground (the output pin)

For duals, National Semi would give the total package supply current, the idea being you couldn't power only one.

Analog Devices gives the supply current per amplifier.

For a buffer with no load, whatever current goes in the top, has to come out the bottom, so total current is 1.05 mA max over temp,

and PD would be 30V * 1.05 mA.

It looks like you are not using the latest datasheet.  (rev. I)

What Fc for your LPF??   I would not go into production relying on a typical input C, or use smaller R and larger external C.

Harry

• Benoit,

The 100k is calculated for a bipolar diff pair.  The capacitances are measured.

For your second schematic, you have the 2.5 pF common mode, but you also have 1.1 pF to AC ground (the output pin)

For duals, National Semi would give the total package supply current, the idea being you couldn't power only one.

Analog Devices gives the supply current per amplifier.

For a buffer with no load, whatever current goes in the top, has to come out the bottom, so total current is 1.05 mA max over temp,

and PD would be 30V * 1.05 mA.

It looks like you are not using the latest datasheet.  (rev. I)

What Fc for your LPF??   I would not go into production relying on a typical input C, or use smaller R and larger external C.

Harry

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