I am finding that the AD8436 RMS-to-DC has a significant startup behaviour - 20+ seconds for my application.
I am using the AD8436 evaluation board: +/- 5 V supplies, IBUF disabled & bypassed, DC coupled, OBUF enabled. I need to measure pulsed signals down to 5 Hz with crest factor > 6, so have added 100uF (tantalum, 6.3V, Kemet T491) to the existing 10uF to achieve the required RMS conversion accuracy. Other components as standard.
After applying the supply power, the output voltage is zero regardless of the input signal until some amount of "startup time" has passed . After the startup time has passed, the output settles to the correct value according to a decay as exemplified in Figure 30 of the datasheet. The startup time seems to vary roughly in proportion with the value of Cavg and in inverse proportion to the input signal applied during startup. For example, for a 20 mV input, the startup time is around 20 seconds. For a 5 mV input signal, the startup time is 70 seconds.
In my application, the input signal will be < 2 mV for 10 seconds while the system boots up, by which time I need the RMS converter output to indicate the correct RMS value. Under these conditions, the AD8436 will still be "starting up".
Currently, I am thinking that I will have to inject some current into the SUM pin for a second or two after power on to bring the startup time below 10 seconds.
Please could you comment:
- Is this the expected behaviour, or is something wrong?
- If this is normal, is there some other way to reduce the startup time?
Thanks in advance,