THS4531: Diff to SE usage

Hello Hideki,

1. For a single supply configuration, it is best to set VOCM to mid-supply, which in this case is 1.5V. This maximizes the dynamic output range of the amplifier. Also, including the 0.22uF capacitor to ground reduces harmonics and minimizes noise from the common-mode which is important when considering single-ended measurements. Or you may leave this pin floating (NC), and it will be set internally to mid-supply. 
2. Power down pin gives the customer the ability to enable/disable the amplifier. To enable the amplifier, the pin must be set to a voltage of 2.1V or greater. When enabled, then the amplifier operates normally at a quiescent current of 230uA per channel. To disable the amplifier, the pin must be set to a voltage of 0.7V or lower. When disabled, the amplifier is in low power-down or shut-down mode, and the amplifier's quiescent current is lowered to 0.5uA per channel. Figures 21 and 22 in the device datasheet shows the differential output in both cases. The customer may set this pin to VS+ if they are not using this functionality. 
3. Leaving output pins of an amplifier floating leads to unexpected results. Best practice is to terminate the output with a resistor to ground matching the load at VOUT+. But, it looks like the customer is AC coupling the output of the amplifier. If they are not space constrained, they could add a transformer at the output of the amplifier to convert the differential signal to a single ended while keeping the FDA's distortion performance. (Shown in Figure 68 from device datasheet, inserted below)
4. Yes they can short VS- to ground. 

Thank you,
Sima