The protecion levels are kind of industry standard. The SDM3065,4065 , HP34401 and most other 6 digit meters have protection via resistance of some 100 K to limit the current. For enough voltage rating the resistors are usually chains of equal values. The specs for the normal voltage input sometimes looks a bit optimistic, with relays that have limited voltage specs. Some meters seem to turn off a relay to prevent over heating the resistors. However his is more like a longer time thing.
For this reason KS has some supension on the specs and use for high voltage.
On the other side with the Sigilent meters the protection for the sense input often looks nearly as good as the main input. So chances are the 200 V specs there would have some reserve. The problem in the ohms more is more the current source that is usually through the main input - this may well not like a full 1000 V.
The question of a ratio measurement is a different thing. They are just reusing the sense terminals. With the 34401 the sense terminals are from the HW side not that different from the normal input. The HP meter use AZ switching at the input and get a kind of differential reading this way. The normal voltage is input and COM=GND or for 4 wire ohms sense H and sense L as the 2 readings in the AZ loop.
Sense low has no pre-charge and thus more switching spike, but would still be an input used for the low side and assumed to be low enough in impedance externally to absorb the current spike. One still wants a way to define the relative voltage between the main input and the 2nd voltage to measure. The direct link from Sense L to COM would be the obvious solution, but there could also be some voltage difference.
With the limited supply in the sigilent meters the COM terminal can not be directly linked to the supply ground. It is very likely driven to the opposite if the main input,as this is the way to get a 20 V high impedance input and at the same time a differential signal to drive the differential ADC.
This limits the available input range between COM and the sense terminals. As an example, if there are 20 V between the normal input and COM, the COM would be at some -10 V relative to the supply and thus little headroom (maybe some 3 V) for a negative voltage between COM and a sense input.
The system with moving the COM signal relative to the supply ground is an elegant way to get a larger range and differential signal, but it is not compatible with easy electronic switching between more inputs. So it would be quite some effort to add an ratio mode to the SDM3065, while it is easy (no extra HW needed) for the 34401.
Home
Help
Search
Login
Register
