OpAmp to uniform amplification of signal from DC to ~100MHz

[gashtaan]

The datasheet is clear; the AD8129/AD8130 inputs, all 4 of  them, must be at least 1.5 volts away from each supply rail, and the output only reaches to within 1.1 volts of each supply rail.  The differential probe level shifts the input into range for the AD8129, and uses a virtual ground for the output which comes out at pin 4 of the connector.

The pin 4 of the connector is optional auxiliary middle level of measured current. The level can be controller either from this pin or by R3 trimmer or even connected to the GND, what they encourage to do if the user want to measure DC with the same gain. But in this configuration +REF will not be 1.1V away from V- negative supply rail. It seems to be some hack that is not following the datasheet.

Can you please point me where in the datasheet is mentioned that all 4 inputs including +VREF must be at least 1.5V away from supply rails?

[David Hess]

Can you please point me where in the datasheet is mentioned that all 4 inputs including +VREF must be at least 1.5V away from supply rails?

Page 5 lists the common-mode voltage range under input characteristics as +/-3.5(3. volts with +/-5.0 volt supplies for the AD8129(30) and a differential operating range of +/-0.5(2.5).  Page 6 lists the output voltage swing under output performance as +/-3.6 volts with +/-5.0 volt supplies.

There are sets of specifications for +5, +/-5, and +/-12 volt supplies which vary slightly.  None of them allow anything like operation close to a rail on the inputs or output.

[gashtaan]

I did see that range on Page 5, but didn't know how to understand that it's meant for specific condition V+IN - V-IN = 0.

In absolute maximum ratings there is:
Input Voltage (Any Input) −VS − 0.3 V to +VS + 0.3 V
Of course, I know it's not OK to push it to these limits

OK, I think I understand now. The documentation (NewAE probe) is somewhat misleading - they forgot to mention that configuration for measuring from DC applies only to double-ended power supply.

[gashtaan]

If I will measure voltage drop across shunt resistor at DC circuit, Vin+ vs Vin- difference would be normally positive, right? If I will use single-ended power supply (5V), set Vref to minimum 1.25V and choose maximal shunt resistance to measure 0-30mA to still keep voltages between allowed range, what will happen in case of short circuit (full 3.3V across shunt)? It will momentarily exceed allowed input voltage, but still be there enough room to not exceed maximum rating.

[David Hess]

If I will measure voltage drop across shunt resistor at DC circuit, Vin+ vs Vin- difference would be normally positive, right? If I will use single-ended power supply (5V), set Vref to minimum 1.25V and choose maximal shunt resistance to measure 0-30mA to still keep voltages between allowed range, what will happen in case of short circuit (full 3.3V across shunt)? It will momentarily exceed allowed input voltage, but still be there enough room to not exceed maximum rating.

Some external input protection can be added to the difference amplifier to protect the inputs from over-voltage.  The datasheet discusses this.

[gashtaan]

Some external input protection can be added to the difference amplifier to protect the inputs from over-voltage.  The datasheet discusses this.

There are discussed cases when power supply being lower than 4V or higher than 12V. Otherwise it is stated:
"If the supply voltage is restricted to less than ±11 V, the internal clamping circuit limits the differential voltage and excessive supply current is not drawn. The external clamp circuit is not needed."

That's why I've asked. If just harmless clipping occurs in cases of over-voltage in either input or output, it doesn't matter to me.

OK, I buy the ICs and test it. I've bothered you enough