### Q

I just got a question from our customer regarding to the application of AD736.They are designing a data acquisition card in which the RMS to DC part is used

to measure the rms value. The power supply for AD736 is +/-15V, the input

voltage range is within 5V and the signal frequency is less than 1kHz.

They would like to know how to choose the proper values for the external

components such as Cav Cc and Cf. Is there any equation to follow?

### A

There is no equation, just the table provided in your email. The averagingcapacitor is connected to a transistor base, and thus dependent on signal

level. Furthermore, the averaging error is inversely proportional to settling

time. I would recommend an empirical approach using an eval board, or I can

recommend a value if the customer could provide a desired minimum and maximum

input level, minimum frequency and maximum rms error. The capacitor value (thus

rms error) matters most at the lower desired operating frequency, and minimum

input amplitude. Figure 16 helps somewhat when estimating the settling times,

but it isn’t accurate because the settling times for rising and falling

transitions are unequal. Falling is about 2x rising.The equation on page 14 is

useful for the low frequency limit. The input at pin1 is 8k +/-20%, the input

at pin 2 is 1012Ω, nearly negligible but a gate-leak resistor may be required

if ac coupling is used, then the resistor value along with the minimum

frequency establishes the capacitor value. In either case, the capacitor value

effect is simply a first order series drop error.One can calculate the 3dB

frequency using 8k and minimum desired frequency to calculate capacitor value.

Bear in mind this filter affects the ripple frequency, which is 2X the minimum

desired frequency.