I'm looking at 2 different optocouplers.The Avaago ACPL-K44T and the ACPL-227.
The ACPL-K43T states that it's a digital opto-coupler while Avago ACPL-227 doesn't.
Schematically, they're a bit different. The K43T needs a Vcc and has parameters like Vol,Voh,Ioh,Iol, etc.
I'm wondering what exactly are the applications you'd need the K44T for compared to the 227.
http://www.avagotech.com/products/optocouplers/automotive/ipm-interfaces/acpl-k43t-000e
Phototransistor-output optocouplers are cheaper and suit for slow digital applications and sometimes for analog applications such as switching power supply feedback. When you calculate the actual switching speed with a reasonably high load resistance (See figure 16) they can be quite slow, but still okay for many applications- such as isolated switches and relay contacts in a PLC. There are also photodarlington optoisolators which are even slower, and a few other types.
Digital output optocouplers are much faster and are specified in terms typical for a logic part, mostly. They use a photodiode on a chip rather than a phototransistor. They also draw power even when off. They are also relatively easy to use (in terms of guaranteeing that they will work under all conditions). You would use them to isolate digital signals, such as for an isolated ADC/front end in a data acquisition system.
Even though most opto-couplers have a digital 'nature' to them (the exception being an H11F1, which has an fet output and is analog over a limited range), there are those that work over wide voltage and current ranges, sometimes referred to as high-voltage digital.
They are designed as a 'logic' interface between 5 volt to 24 volt digital logic, with outputs that can work up to 200vdc (H11D1). Data rate is limited to a few hundred KHZ. They are use for machine control and slow servo control loops. Your ACPL-227 is a typical general purpose opto-coupler.
A digital opto-coupler works at standard 3 volt to 5 volt logic voltages, and expects a clean logic type drive current in order to output a clean logic signal. Some can run as fast as 10Mbps, and will likely get faster over time. Your ACPL-K43T is like this, for fast logic (data) transfer or a fast isolated power switch in terms of machine control.
ACPL-K43T has a digital output which means you can directly connect that output to any logic circuit operating in TTL levels. ACPL-227 has a bare transistor output: you will need to provide an external resistor and voltage source to match your output requirements. ACPL-227 offers multiple optocouplers in a single package that can be customized while ACPL-K43T offers convenience for a specific application.
Digital optoisolators are designed for speed and on/off outputs, regular optoisolators are designed to be more analog so you can send a range rather than just an on/off signal (speed is a secondary concern). That's the main difference.
