Full wave rectifier using opamp - 0 to 5v ac (for measuring voltage drop across wire, required for Earth Contact Resistance Tester design)

Here is an app note:

Precision Full-Wave Rectifier, Dual-Supply

I might substitute a chopper low offset zero drift op-amp.

I initially considered using the OPA2211, as it fits the performance requirements, but it's not cost-effective for my overall project. I'm aiming to design an affordable ECRT tester that fits within a panel meter form factor.

Any affordable & readily available opamp suggestions ??

Diodes Incorporated AS333SE-7

single supply rectifier....

You could just ditch the op-amp precision full-wave rectifier, and just sample the unrectified signal, since the ADS1115 supports 860sps. Also, I presume you're going to to do it synchronously, since you know the phase of the AC signal. Sure the resolution will be affected but I'm not sure how much that will actually matter compared to the more complex circuit with rectification.

If I ditch the op-amp, since the ADS1115 can't read negative voltages, I need to bias the AC signal to sit within the ADC's range & calculate RMS and peak from the sampled signal in software. 

is this the correct approach : AC signal → capacitor → ADC with 2.5 V bias ? & what kind of protection should in input.

Yes, perhaps prototype that or simulate the input and give it a shot. You might still need a buffer but you can see what works. Check the datasheet to see what input protection exists, it will have advice in either the ADC datasheet or whatever op amp you're considering. 

You can't assume that the excitation current is stable since the mains voltage will vary and the system  impedance will affect the current. The contact resistance will be non linear.

There is no problem with AC coupling the signal to the amplifier but you will need a low pass filter before the ADC. 

You will need to measure alternate samples of the excitation current and the voltage across the contact to get accurate results (although 10% of reading is usually good enough for contact resistance measuring).

Your Arduino can have a 12 bit ADC on chip (depending on model of Arduino) so you may not need the ADS1115 at all which will reduce the cost. You can do much better (in size, cost and performance) if you use a micro-controller outside the Arduino environment. 14 bit converters, multiplexer sequencing and much faster conversions are easily available. You will have issues with the I2C interface of the ADS1115 if you try to run it at full speed with an Arduino - but you may have reasons for using one that I don't know 

MK

Thanks for the detailed insight, MK 

Regarding your suggestion to move outside the Arduino environment for better size, cost, and performance — could you please recommend specific microcontroller that offer integrated 14-bit (or better) ADCs, support multiplexer sequencing, and allow faster conversions ?

I'd be keen to explore alternatives that meet these requirements.

Thanks again for your time and valuable input!

The processor I had in mind was one I've used recently - STM32U575CIT6. The cheapest version of this family is the STM32U575CGT6 available from Mouser at £3.99 (100 off price).

It's a 48 pin LQFP package and it has 768kbyte RAM, 1M flash and clocks at 160MHz max. All the info at ST website.

The development board for this micro is about £19 from Farnell https://uk.farnell.com/stmicroelectronics/nucleo-u575zi-q/nucleo-144-board-32bit-arm-cortex/dp/3798138?cfm=true

The processor has a lot of code security features that you don't need to bother with unless you need them, and is supported by free dev tools from ST (although I used Keil ARM tools on the project).

I'm not convinced that you really need the 14 bit ADC - processors with 12 bit ADC are a lot cheaper.

There may be other ST processors with the 14 bit ADC that are cheaper.

MK