is mux-friendly and EMI hardened on its input. I wonder how mux friendly it is on its output. The compensation scheme (fig on page 18 of the data sheet) has Ccomp hanging on the output pin, like LM8261 which is c-load stable and pretty good driving multiplexers.
Many opamps freak out driving multiplexers. That can cause weird errors and channel crosstalk effects.
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John Larkin Highland Technology, Inc
lunatic fringe electronics
What's nasty is that most mux's and multiplexed ADCs act like a charge storage node that's switched between channels, and most opamps freak out for a when when a slug of charge is injected into their outputs. Some ring badly, some just jump and take a while to recover.
All cmos mux's add extra charge injection, from the gate drives to the cmos switches, from
That could be caused by output impedance too high; for an LM324, the usual treatment would be a pulup or pulldown resistor (so the output is never unloaded). There's some multiplexers that have inbuilt load/termination resistors, too, so the solution doesn't have to include offchip components.
Need to make a composite amplifier with a chopamp for DC and LF, and a wideband for RF. The problem will be at the crossover. The wideband will have much less prop delay so there will be a phase glitch near the crossover.
Not knowing what charge injection to expect (but probably negative, because P-channel capacitance during turnon is likely higher than N-channel for a CMOS analog switch), it matters if the resistors are pullup or pulldown. To keep emitter impedance on the output low, you want to ensure a healthy emitter bias current from the NPN emitter in a complementary class A/B follower output stage, to minimize the glitch with a transient sink of current. That'd be a pulldown resistor, to some supply below your signal level.
The double-damned modern datasheet on the OPA2189 doesn't give a clue of the output driver nature; maybe it's CMOS? It could be tested, of course. Feed a square wave through a small capacitance into the output pin, and see if the edge that best approximates a MUX channel turn-on transient is improved by load resistor to +V, -V, or ground.
Series resistance, and a bit of load capacitance, also decouples the op amp output, but utilizing the internal compensation capacitor seems like the cleaner solution.
I'm thinking of chips like the Hittite HMC1022ACHIPS, 48 GHz broadband amplifier. The output is AC coupled and I would like to add an additional amplifier to restore the DC component. See Fig. 47 in
OK, we've thought of that. You need a splitter ahead of the amp pair and a combiner, basically a bias tee, at the output.
I don't think the crossover is terribly difficult, at least in theory/simulation. The crossover could be low, KHz to low MHz. But the details will be difficult at 48 GHz.
Are you going to wire-bond that thing?
You could also maybe DC couple that chip, with appropriate supply offsets and such, and avoid splitting/combining. There would be one offset step to manage somewhere.
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John Larkin Highland Technology, Inc
lunatic fringe electronics
The two fets are cascaded. The lower one sets the current. The upper one is the RF input and is biased at +4V. The output signal is at VCC and is AC coupled to the load.
You could bias the lower one at -4V and the upper one at GND. The output is now at VCC - 4V and would have to be AC coupled. So it would still require DC restoration to recover the DC value at the input.
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