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ISPPAC81 Datasheet(PDF) 11 Page - Lattice Semiconductor |
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ISPPAC81 Datasheet(HTML) 11 Page - Lattice Semiconductor |
11 / 20 page Lattice Semiconductor ispPAC81 Data Sheet 11 JTAG User Bits There are a number of user-configured E 2 bits that control various aspects of and can all be accessed in either the 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 2 bits available is listed in Table 3. 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- nected to a DC bias, preferably VREFOUT. The input signal must either be AC coupled or have a DC bias equal to the DC level of the other input. Since the input voltage is defined as VIN+ - VIN-, the common mode level is ignored. 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 DC level of the single-ended output will be VREFOUT. If the load is not AC coupled and is at a DC potential other than VREFOUT, the load draws a constant current. Using one of the differential outputs halves the available output 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. The input common-mode voltage is VCM = (VCM+ + VCM-)/2. When the value of VCM is 2.5V, there are no further 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. When VCM is not 2.5V and the gain setting is greater than one, distortion will occur when the maximum input limit is reached for a particular gain. The lowest VCM for a given gain setting is expressed by the formula, VCM– = 0.675V + 0.584G·VIN where G is the gain setting and VIN is the peak input voltage, expressed as |VIN+ - VIN–| and the highest VCM is VCM+ = 5.0V - VCM– where 5V is the nominal supply voltage. In Table 4, the maximum VIN for a given VCM– to VCM+ range is given. If the maximum VIN is known, find the equiv- alent or greater value under the appropriate gain column and the widest range for VCM will be found horizontally across in the left-most two columns. Only a VCM range equal to or less than this will give distortion-free perfor- mance. Conversely, if the maximum VCM range is known, the largest acceptable peak value of VIN can be found in the corresponding gain column. All values of VIN less than this will give full rated performance. |
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