Why are oscilloscope input impedances so low?
A lot of things are the way they are because of history, and de facto standardisation.
A general purpose oscilloscope input is a difficult compromise between not loading the circuit, not being damaged by high voltage, having reasonably low noise, and being able to maintain a decent bandwidth.
1Mohm in parallel with 15pF to 30pF satisfies a lot of people for a lot of applications. There's little incentive for manufacturers to build a general purpose oscilloscope with a different input, to address tiny parts of the market.
When you do need better noise, or a differential input, or a higher input impedance, then you use a custom pre-amp. When you need wider bandwidth, you switch to a 50 ohm input impedance.
There are special purpose oscilloscopes made at high prices that do address niche applications.
I would say a combination of a few factors.
- The input stages of an osciloscope are a difficult compromise. They need to be have a wide range of gains/attenutations, they need to be tolerant of user errors, and they need to pass high bandwidths. Adding a requirement for a very high DC resistance would just further complicate matters. In particular attenuators needed to handle the higher end of the scopes input level range would get much more complex/sensitive if they needed to have a very high DC resistance.
- It's a de-facto standard, changing to something else would lead to incompatibilities with existing probes etc.
- There wouldn't be much benefit anyway.
To further explain point 3, at moderate frequencies (from a few kilohertz upwards) the 1 megohm DC resistance of the scope input is not the dominant factor in the overall input impedance. The dominant factor is the capacitance, with the cable making probably the largest contribution.
(in fact at UHF/microwave frequencies it's common to reduce the scope input impedance to 50 ohm, so the inductance in the cable can balance out the capacitance and the cable becomes a properly matched transmission line)
What this means is if high input impedances are desirable then it's much better to deal with that at the point of probing than at the scope. The typical compromise of cost/flexibility/input impedance for general use is an x10 passive probe.
If you need a really high DC resistance then the solution is to add a FET based amplifier in front of the scope, preferably as close to the point of measurement as possible.
Actually, it is ridiculously high for a wideband input.
There is no practical connector or cable that actually has an impedance (from a transmission line view. Resistance, but for coaxial cablers, gold platers, and waveguide plumbers. RF dudes.) of 1 megaohms, leaving the input utterly mismatched - even worse, a 15-45pf capacitor across an 1 megaohm (transmission line impedance) input would mismatch it to oblivion.
The reason it is 1 megaohm is for supporting standard 10:1 probes, which you indeed need to not overload the kind of circuit carrying audio frequency signals at high impedance and with high DC offset (think audio vacuum tube circuits, the probe designs are from just that era).
However, once you are dealing with RF or fast digital circuitry, the parallel capacitance of the scope input (which you can't make too small, again because of probes, cables, connectors) will dominate ... and bring the actual input resistance of that input down to 5 to 10 kiloohms once you reach one megahertz, 500 to 1000 ohms once you reach 10 megahertz. Reach VHF (hint: ACMOS or F-TTL circuitry is VHF stuff even if you don't clock it at VHF) and you would be better off with a matched 50 Ohm input, since you could connect a (within reason) long 50 Ohm cable and still have a 50 Ohm input on the circuit end, instead of an even bigger capacitive burden.
With the conventional kind of probe and input, you will overload RF circuitry easily. RF optimized oscilloscopes tend to have inputs that can be switched to 50 Ohm input impedance (any oscilloscope input can, with a parallel/through terminator) - which is, interestingly, BETTER suited, since now you can use probes (eg Z0 probes or active FET probes) that actually can be made to present much higher effective input impedances at the probe point. Or just provide a reliable 50 Ohm connection to your circuit with any old RG58 cable.