Hi, thanks for your prompt reply.
I've been given a talking to and I've calculated that the impedance is about a couple of k. So this should be ok. I've got a 3.9k, 10k and 15k effectively in parallel after the main circuit (which is pretty big, oscillator,reciever,sum&difference multiplier and differential amp).
The main problem is that the A/D seems to sample for a short period of time, and then stop. The it seems to stop when there's a large change in values.
Similar potential divider sensors such as my LDR and thermistors run for a good while, but the ultrasonic cicuit mentioned above seems to stop after just a couple of seconds.
I'm debugging by displaying the values on an LCD, and it just holds the last value, and needs the development boards reset button to be pressed before it goes again.
Matt