# Frequency design parameter for DC signals

I have been using some online tools to design a circuit to amplify photo-diode current. I have understood that all such tools needs a frequency/bandwidth to drive the calculations.

For example, a Low Pass circuit has a "frequency" parameter in it. OP Amps describe there performance for different frequencies but I am interested in a DC voltage. The tools forces to choose frequency greater than 0. Why is this?

Example of tools:

http://www.analog.com/designtools/en/filterwizard/

http://sim.okawa-denshi.jp/en/CRlowkeisan.htm

• How fast can your photodiode current change? What light intensity changes are you interested in? – Andy aka Mar 12 '15 at 14:13

You may believe in your heart that you want to amplify a photodiode current that remains perfectly constant. I'd make the point that if it remains perfectly constant then why amplify it and measure it?

Of course you need a bandwidth when designing a filter even if it is 1Hz or 0.1Hz or 1MHz.

There is no such thing as a DC filter.

Even pure DC voltages can have an AC component to them. This is particularly evident when the voltage changes quickly, such as when a digital output changes from low to high or vice versa or when a photodiode has an increase or decrease in the amount of light that hits it. The low pass filter merely shapes the waveform produced by the change in voltage.

• OK I understand but then what value should I use in the tools? I am currently punching "1" Hz to drive the parameters. I like to think in a way that the capacitor is absorbing the sudden change rather than allowing it to pass to the ground. – Anuj Purohit Mar 12 '15 at 6:38
• That completely depends on what slew rate you want to support and what component values are cost-effective. – Ignacio Vazquez-Abrams Mar 12 '15 at 6:40
• For sudden change in DC voltage, I like to think in a way that the capacitor is absorbing the sudden change rather than allowing it to pass to the ground. Is this correct or does it allow it to pass? – Anuj Purohit Mar 12 '15 at 6:41
• Capacitors block low frequencies and pass high frequencies. – Ignacio Vazquez-Abrams Mar 12 '15 at 6:43
• A 1Hz bandwidth for a first order filter will take about 5 seconds to settle to 1%, 10 seconds to 0.01%. Do you want it to take that long? – Kevin White Sep 9 '15 at 23:31