Question
I just got a question from our customer who was using AD8130 in their project.
They found a problem when using this amplifier in both differential input and
single end input. In the datasheet, we see that if differential input is used,
the operating range is +/-2.5, which means that for each input signal, Vin+ or
Vin-, there will be a +/-1.25Vp-p maximum amplitude. And if RF=RG , resulting
in Gain=2, the output voltage should be clipped as +/-5V. This function has be
proved by our customer. However, when they use this circuit in single end mode,
connecting Vin- to GND and adding a 10Vp-p (or say +/-5V) signal to Vin+, the
input signal can be amplified to +/-10V at the output. That is to say, the
equivalent differential voltage range was two times of the specification. We
are not sure if there is any mistake in our understanding the spec.
Could you please help to give some comments on this issue.
Answer
Balanced 1.25Vp-p signals on each input comprise a 2.5Vp-p differential signal,
which, when multiplied by G = 2 should give close to 5 Vp-p at the output on
+/-12V supplies with the REF input grounded. (It looks like they have an EQ
stage that will boost the gain at high frequencies.) There is a diode limiting
circuit on the input that starts to cut in at about 2.5V, so there may be some
distortion and/or reduction in amplitude in the output signal. If Vin- is
connected to GND and +/-5V is applied to Vin+, the input will clip due to the
limiting circuit – please see the differential input range in the specification
tables and Figure 118 of the data sheet.