AD8546 'resistive load' limits

Recently I needed to specify a dual opamp with uncritical specs, for use in an oven controller:

(The other half of the amp interfaces the thermistor to the ADC, not shown.) 

A quick parametric search suggested that the AD8546 would be a good fit. Now that I'm looking closer at the data sheets prior to ordering, though, I'm seeing an unfamiliar warning:

What kind of opamp has problems driving resistive loads, and at the 100K level?!  Is this a misprint?  Nothing in the output impedance or short-circuit current specs suggests any limitations along these lines:

So I wonder if I'm missing something important here.  Any thoughts?  Obviously there are plenty of other opamps that will work in this application but before rejecting this one I'd like to learn more about the reasoning above.

  • 0
    •  Analog Employees 
    on Jun 25, 2018 12:05 PM

    Hi,

    AD8546 is a micropower opamp with 22uA maximum supply current at 18V designed and intended to be used for low power applications such as portable medical equipment and current loop drivers. Since it is low power, it is the reason it is rated to drive resistive loads down to 100kohms. Can I know what specs are important in your design?

     

     

     

     

     

     

     

  • The specs aren't the least bit critical, given unity-gain stability and the ability to run from +5V, as long as the part has the same footprint as the AD8546.  I'm sure the AD8546 will work OK in my (closed-loop) application.  I'm mostly just curious as to the conflict between the 11 mA Isc / 15-ohm Zout specs and the whole notion of being unsuited for driving resistive loads under 100K.

    How do you reconcile the Zout spec with figure 27, for instance, which shows a 0.3V drop to the supply rail at a load current of 1 mA?  That'd be more like 300 ohms, wouldn't it? 

    You guys may consider it a specialized "micropower" part, but it's listed at DigiKey as a "General Purpose" device with perfectly reasonable specs:

    So the bug may be on the marketing side.

  • 0
    •  Analog Employees 
    on Jun 28, 2018 12:39 PM

    I think you are referring to Figure 18 or 19 instead of Figure 27 which shows the output swing range of AD8546. These figures demonstrates the ability of the output of the opamp to swing while delivering output current which is then related to the allowable output load that it can drive. In this figure, there is 0.3V drop to the supply rail at a load current of 1 mA as you mentioned in your reply which means that your output voltage high for example is already above the minimum headroom of 30mV. Beyond this point, your output voltage will continue to degrade until the point of Isc 12mA which is the maximum current.

    Note that most of the AD8546 Electrical Characteristics are specified with RL equal to 100 kohms or 1Megaohms because at this points output swing is still below or equal to the headroom 0.01mV to 30mV and load current is low from 0.001mA to around 0.18mA. Load resistance for example at 0.18mA and headroom of 30mV is around 100kohms (17.97V/0.18mA). 

    Meanwhile, the closed loop output impedance (Zout) of AD8546 is described in Figure 32 "Output Impedance vs Frequency" of the datasheet. This graph shows that the Zout changes as you sweep frequency and your Zout is dependent on what frequency you will operate. Zout is dependent of many factors such as for example the internal output structure of the opamp. To simplify using Thevenin's theorem, it is like a source resistance in series with the load.

    Regarding Digikey, I think they just categorize the amplifiers in their website like in the image below. The classification type is not so specific for example "low noise amplifier" or "low power amplifier". They have their way of classifying the amplifiers they distribute but we don't have control about that.