i saw the ad620 amplifier used in the digital scale/strain gauge walkthrough. they are areound $7 each when bought individually, just wondering if there was a cheaper equivalent that anyone knew about?

thanks, rob

I have had good luck with the Op07 chips. Also from Analog Devices, it has been around for a long time. Advantage over many chips, it is designed to function well on a single-ended power supply with no special mods.

interesting. i read the Op07 data sheet, but i'm still too new to understand all the ins and outs.

the ad620 allows the user to program the gain with a resistor (i believe the nerdkit tutorial video mentioned they had the gain set at 150x if my memory serves me correctly). i didn't see anywhere in the Op07 datasheet on how to control the gain. am i missing that, or is the Op07 set at a static gain?

thanks

OK, the 620 has that advantage. It's set up as an instrument amplifier, whereas the Op07 is a basic amp. Instrument amps actually contain several amps in a configuration that allows for the kind of use that the single chip is probably better at.

The 620 is an all-in-one unit. To make a similar circuit of equal function, you would need three or four Op07 chips. Because it would have to be built from discreet components, the error levels will be a bit higher. Budget-wise, it would make better sense to go with the 620, in space, component count and probably cost too.

so what your saying is.... the Op07 isn't really a cheaper equivalent?

not a huge deal. i guess i'll give the ad620 a try then, even though its a little spendy.

It really depends on what you want to do with it.

Instrument amps are generally high tolerance chips and usually beyond what a typical hobbyist might need. The Op07 chips are a good work horse for simple yet still very good circuits if you don't really need such accuracy. I have used them for years in audio and other signal circuits with very few difficulties. The gain is still a simple setup on OpAmps even though there might be a couple of resistors and some math involved.

I have used the instrument amps in circuits like high reliability temperature controls and displays but that is where they really shine.

gotcha. thanks for the info.

If you're willing to wait a bit, I've ordered a bunch of components from furturlec.com. They send your order from Bangkok, Thailand though so it does take about 3 weeks to arrive. Also, they do have the AD620 for $5.00, so I dunno if a $2 savings is worth the wait. Just throwin it out there.

Ted

I recently bought some strain gages and tried to do as the tutorial says, but without the AD620.

Instead, I use a regular LM358. First trade off: I'm unable to make it work with an alternate excite, like the one provided in the code C.

Second, for an unknown reason (to me), I can't get very high gain. It seems something happens when I try to obtain more than a few hundreds in gain that simply kill the signal. Too much noise perhaps?

Anyway, if you don't need very high precision, an ordinary op amp will work just fine. Simply make sure you change the program to analyze a one way excite, which will liberate two pins on your MCU.

Good luck!

Frozenlock

An instrument amplifier is very sensitive, and very high impedance (doesn't load the circuit feeding it). There are many equivalents for the AD620. The one I recommend (because it's equivalent, and costs far less) is this one:

INA126P by Texas Instruments (or a subsidiary they bought)

You can find it at Mouser for $2.70 in quantities of 1.

You can also use a MAX4194, if you happen to have one lying around. I haven't price checked it.

Cheers BM