AD694: Bandwidth of buffer and IC

Q 

Inquiry Description:
Hello!

I'm using the AD694 4-20mA Transmiter and I would like to know its bandwidth.
I've read the datasheet and it says that the buffer amplifier has a frequency
response of 300 kHz, but I'm not sure if this applies to the whole IC.

I've been testing this IC and so far it works fine up to 10 kHz, beyond that
frequency the signal is attenuated and in the end it is distorted.

However, my intention is to transmit an up to 100 kHz signal and I'm not sure
if the AD694 is the correct choice. I would apreciate if you could give me any
insight about this?

A 

The frequency response of 300 kHz (typ) given in the specification section of
the www.analog.com/AD694 datasheet applies to the buffer amplifier.

Please remember, that the frequency response is specified for small signals.
Small signals are usually 200 mV p-p.

However, the limit to the bandwidth specification of the AD694 is determined by
the output characteristics. Kindly refer to the output characteristics in the
Specifications section of the datasheet, page 2.

The typical slew rate of 1.3mA/us for rising and falling signals along with
specified current output range of 4mA to 20mA result in a rise and fall time
of:  
t_rise,fall = output range / slew rate = (20mA-4mA) / 1.3mA/us  =  12.31 us.

With the maximum switching assumed to be 2 times the rise/fall time, that makes
the maximum bandwidth limited by slew rate and output current as:
BW_max = 1 / (2 x t_rise,fall) = 1 / (2*12.31us) = 40.6 Khz.

So the actual slew rate and the current drive range in the application will set
the maximum bandwidth of the AD694.

Please remember, that the characteristics given in the Specifications section
of the part's datasheet are specified under certain conditions. In case of
AD694 the characteristics are specified for a load of RL = 250 Ohm and a supply
of Vs = 24V.

For parameters differing from these conditions the characteristics might
change.