Digitizer CLK IN signal type?

[V2000]

Hi! I'm using an NI-5771 digitizer with a PXIe-7975 FlexRIO FPGA module. I apologize for the newbie questions, but it is not quite clear to me what type of input signal is expected for the CLK IN input on the digitizer: Is it a bipolar, AC sine or a logic-type signal?

The specs sheet for the NI-5771 specifies the following:

Number of channels..........................................1, single-ended

Connector type..................................................SMA

Input impedance................................................50 Ω

Input coupling...................................................DC

Input voltage range ...........................................0.9 Vpk-pk to 3.5 Vpk-pk

Absolute maximum voltage without damage ...10 Vpk-pk

Duty cycle.........................................................45% to 55%

This sounds to me as if a bipolar sine signal is required (e.g. a sine with 0V offset and 2Vpk-pk amplitude would work), but I would be grateful for a confirmation. What is the threshold voltage that defines a clock transition – I'm assuming, zero volts?

In other words, a logic-type, unipolar square wave clock signal, e.g. TTL (with V_L ~= 0.2V and V_H = 5V) or LVCMOS (3.3V), won't lead to a reliable registration of my input clock? If this is the case, can anyone recommend a simple and affordable solution for converting from a logic-type clock to a clock that is compatible with the CLK IN input? Would it be enough to add a negative bias to a TTL signal, to make sure that its rising edges reliably cross zero volts?

Thank you!