ICs that convert an AC signal into its true rms value
Digikey has two pages of ICs that form the basis of a system to do this here 

The examples below are RMS voltage to DC converters. Current to DC conversion requires an initial "simple" current to voltage conversion.   Care needs to be taken re crest factors that the IC's will work over (over about 5:1 needs special care) and accuracy reduces with increasing crest factor. Input voltage range may be limited eg 5 to 1000 mV for AD737, less again for LTC1966. ie - while this sort of IC is useful the limitations need o be appreciated and the datasheet should be carefully examined.



Gargoyling  

         rms converter  


as per here will produce a large number of references .



Just to open up the mental filters - you can use thermistors and "ordinary" opamps as the heart of an RMS converter as seen here.



And to carry this to extremes [tm] the LT1088 integrates two heaters and temperature driven diodes in an IC package and provides an RMS converter with a 3dB bandwidth of 300 MHz (!!!!), 50:1 crest factor (!!! again) but a still only modest 20:1 dynamic range.
 See Linear Technology AN 22 - 1987!!! - A Monolithic IC for 100 MHz RMS-DC conversion.
 You mau have trouble BUYING such an ICnowadays, but the ideas in the app note would allow you to buld a creditable one with "not too much work" [tm, yet again].

Yee Ha !!!





Cheap modern off the shelf entry level products:

As an example only - chosen on the basis of being in stock and chea[pest in 1 quantity - LTC1966  - datasheet here  $5.88/1 in stock. Looks "useful. They say 


Simple to Use, Requires One Capacitor
True RMS DC Conversion Using DS Technology
High Accuracy:


0.1% Gain Accuracy from 50Hz to 1kHz  
0.25% Total Error from 50Hz to 1kHz  

High Linearity - 0.02% Linearity allows Simple System Calibration
Low Supply Current - 155µA Typ, 170µA Max
Ultralow Shutdown Current - 0.1µA
Constant Bandwidth independent of Input Voltage - 800kHz –3dB, 6kHz ±1%
Flexible Supplies: 


2.7V to 5.5V Single Supply  
Up to ±5.5V Dual Supply

Flexible Inputs: Differential or Single-Ended
Rail-to-Rail Common Mode Voltage Range
Up to 1VPEAK Differential Voltage
Flexible Output: Rail-to-Rail Output
Separate Output Reference Pin Allows Level Shifting
Wide Temperature Range: –55°C to 125°C
Small Size: Space Saving 8-Pin MSOP Packag e


For $8.24 /1  the AD737 datasheet here comes in a PDIP8 package, seems a t  aquick skim to offer similar perormance but with some pros and cons. (Made for ADC interfacing, multimeter focus, with or without output buffer (lower offset voltage without), ...

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Q&amp;A

Q:  What you suggsted above is a RMS to DC converter.   

I need an IC to convert my instantaneous AC signal to its true RMS value. – george 36 mins ago

A:  An "instantaneous" AC signal IS a DC level. 

The RMS value of an AC signal over a period is a DC level over the same period. 

What may be wanted is the RMS value of an AC signal NOW based on the signal for a prior period up until now.   

The DC value of such a calculation may change at each calculation and so give an AC varying RMS value.