I have an existing design that uses an AD8138 for single ended to differential conversion to drive an ADC with 499-ohm RG and RF, and 1 input leg shunted with 49.9 ohms to match a 50ohm single ended input source.
I have a customer who wants a 10K ohm or greater input impedance and can accept a bandwidth reduction down to about 20MHz . My suspicion is that lots of bad things, like noise, and offset errors will increase with higher impedance's...
1) In a unity gain configuration, how high can I increase RF & RG before the amplifier becomes unstable?
2) Is there a relationship between input impedance and bandwidth as well? , he can live with a 20MHz bandwidth?