Looking for op-amp solution with offset and high-z input impedance

Hi,

I'm building a modular synthesizer system which has a number of inputs that should be able to accept DC coupled voltages from -5 to +5, which I need to convert to the input range of my codec ADC, which is 0 to 3.3 volts.

I designed a simple inverting op-amp circuit with offset compensation like this, (VREF=1.24V) which basically does what I want. Input voltage of 0V results in 1.65V on the output, which is nicely halfway the supply voltage of 3.3V.

I forgot about one thing though: the user is able to connect and disconnect the cable on CONN7, and when it is unplugged it of course goes high impedance. In this case the output of the op-amp is not the same as when it is connected to 0V because the voltage on the negative input will then always be pulled to VREF.

I've been looking for a simple fix, but I can not figure out how to get this to work with one op-amp only.

So what I need is basically this:

• Input range -5 .. +5 V
• output range 0 .. 3.3V
• Inverted or not inverted, does not matter
• Input impedance approx 10K
• Input not-connected should behave as if input is connected to 0V

Are there an obvious solutions to this that I'm missing?

Thanks!