I need to design a log amplifier with the following specs:
- DC-1Mhz bandwidth min. DC working is a must
- 120 dB dynamic range with 5uV to 5V input range (-106dBV to 14dBV or -93dBm to 27dBm)
- The input is differential and one of the input is is fixed 3.3V and the second one ranges from 3.3V to 3.3+5=8.3V
The log output can be single ended and will be connected to an ADC
If the input of the differential log amp is not an high impedance amp, it’s enough if it would have a constant impedance (+/-5% max) with a very low GND and VCC leakage current, in the order of some nA max.
I saw the combination of the AD8307 + the front-end AD603 (see the AD8307 rev B on page 16 the basic idea). However I have some doubts:
- The input of the AD603 is not differential
- is not clear if the AD603 works from the DC . It is specify from 100Khz only and the AD8307 wors from 10Hz only
- connection from AD8307 and AD603 is single ended and not differential. What is the impact of the noise?
- It is not clear how to manage the -106dBV to 14dBV input range with 3.3V common voltage
Any idea how to solve my design problem ?