throbscottle
Well-Known Member
I'm struggling somewhat to get my head around frequency compensation caps.
Scenario 1. If I have a resistive potential divider of several resistors which can be connected at different nodes to the load (an op-amp + input) using reed switches.
Scenario 2. If I have a resistive potential divider where there are only 2 resistors, but one of them can be switched for different values by reed switches. The common point of the switches is connected to the load (again an op-amp + input)
The top resistor is 9M in scenario 1 or 10M in scenario 2
I know each resistor has some inductance and some capacitance, and that the switches have some inductance and capacitance, and the load circuit has some capacitance (and resistance high enough to ignore). I also know that the ratio of reactance should be the same as the ratio of resistance.
Now, given that I'm somewhat optimistically hoping for a -3db point around 1MHz, how in 7 hells do I work out what caps I should be putting in? Is it as simple (eg if the divider is 10:1) as putting in capacitance of say (50 - parasitic)pF on the lower part of the divider and a 5pF trimmer on the top part? (which is already giving horrendous drop in top resistor's impedance at 10KHz) Or do I have to do complicated maths?
edit: maybe I should just be putting a trimmer in the bottom half
Scenario 1. If I have a resistive potential divider of several resistors which can be connected at different nodes to the load (an op-amp + input) using reed switches.
Scenario 2. If I have a resistive potential divider where there are only 2 resistors, but one of them can be switched for different values by reed switches. The common point of the switches is connected to the load (again an op-amp + input)
The top resistor is 9M in scenario 1 or 10M in scenario 2
I know each resistor has some inductance and some capacitance, and that the switches have some inductance and capacitance, and the load circuit has some capacitance (and resistance high enough to ignore). I also know that the ratio of reactance should be the same as the ratio of resistance.
Now, given that I'm somewhat optimistically hoping for a -3db point around 1MHz, how in 7 hells do I work out what caps I should be putting in? Is it as simple (eg if the divider is 10:1) as putting in capacitance of say (50 - parasitic)pF on the lower part of the divider and a 5pF trimmer on the top part? (which is already giving horrendous drop in top resistor's impedance at 10KHz) Or do I have to do complicated maths?
edit: maybe I should just be putting a trimmer in the bottom half
Last edited: