Question about AD834 + AD811 as voltage controlled amplifier
Vg can be a differential input - if you look carefully they also suggest grounding Vg-, then Vg+ becomes a dc level relative to 0V that controls the gain of the device.

I also think it is a typo on page 15 - with zero control voltage (Vg- at 0V and Vg+ at 0V) there is a big attenuation (70dB) of the real input (Vin). When Vg+ is 1V greater than Vg- the voltage gain of the circuit is +12dB.