Lattice Semiconductor

ispPAC81 Data Sheet

11

JTAG User Bits

There are a number of user-configured E

pull-down menus or directly in the schematic design entry screen of the PAC-Designer software interface to the

ispPAC81. See the online help associated with the ispPAC81 in PAC-Designer for more details of how to set/pro-

gram various operation modes. The list of control E

Differential I/O

Differential peak-peak voltage is determined by knowing the signal extremes on both differential input or output

pins. For example, if V(+) equals 4V and V(-) equals 1V, the differential voltage is defined as V(+) - V(-) = Vdiff, or

4V - 1V = +3V. Since either polarity can exist on differential I/O pins, it is also possible for the opposite extreme to

exist and would mean when V(+) equals 1V and V(-) equals 4V, the differential voltage is now 1V - 4V = -3V. To cal-

culate the differential peak-peak voltage or full signal swing, the absolute difference between the two extreme

Vdiff’s is calculated. Using the previous examples would result in |(+3V) - (-3V)| = 6V. It can be immediately seen

that true differential signals result in a doubling of usable dynamic range. For more explanation of this and other dif-

ferential circuit benefits, please refer to application note number AN6019, Differential Signaling.

Single-ended Input

To connect the ispPAC81 differential input to a single-ended signal, one of the differential inputs needs to be con-

The signal information is only present on one input, the other being connected to a voltage reference.

Single-ended Output

Connecting the output to a single-ended circuit is simpler still. Simply connect one-half of the differential output, but

not the other. Either output conveys the signal information, just at half the magnitude of the differential output. The

voltage swing (3Vp-p versus 6Vp-p). If the load requires DC current, the available voltage swing is reduced. The

output is capable of 10mA, so any DC current raises the minimum allowable load impedance.

Input Common-Mode Voltage Range

For the ispPAC81, both maximum input signal range and corresponding common-mode voltage range are a func-

tion of the input gain setting. The maximum input voltage times the gain of an individual PACblock cannot exceed

the output range of that block or clipping will occur. The maximum guaranteed input range is 1V to 4V, with a typical

range of 0.7V to 4.3V for a 5V supply voltage.

input restrictions other than the previously mentioned clipping consideration. This is easily achieved when the input

signal is true differential and referenced to 2.5V.