PAC80 Datasheet, PDF (11/19 Pages) Lattice Semiconductor – In-System Programmable Analog Circuit

Specifications ispPAC80
Theory of Operation (Continued)
Differential I/O. Differential peak-peak voltage is deter-
mined 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 calculate the differential peak-peak
voltage or full signal swing, the absolute difference be-
tween 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 differential circuit benefits,
please refer to application note AN6019.
Single-ended Input. To connect the ispPAC80 differen-
tial input to a single-ended signal, one of the differential
inputs needs to be connected to a DC bias, preferably
VREF_OUT. 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 V_IN+- V_IN-, 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 VREF_OUT. If the load is not AC
coupled and is at a DC potential other than VREF_OUT, the
load draws a constant current. Using one of the differen-
tial outputs halves the available output voltage swing
(3V_p-p versus 6V_p-p). If the load requires DC current, the
amount available for 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 ispPAC80, 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 an
extended typical range of 0.7V to 4.3V for a 5V supply
voltage.
The input common-mode voltage is V_CM = (V_CM+ + V_CM-)/2.
When the value of V_CM 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 V_CM 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 V_CM for a given
gain setting is expressed by the formula, V_CM– = 0.675V
+ 0.584G·V_IN where G is the gain setting and V_IN is the
peak input voltage, expressed as |V_IN+ - V_IN–| and the
highest V_CM is V_CM+ = 5.0V - V_CM– where 5V is the
nominal supply voltage.
In Table 4, the maximum V_IN for a given V_CM– to V_CM+
range is given. If the maximum V_IN is known, find the
equivalent or greater value under the appropriate gain
column and the widest range for V_CM will be found
horizontally across in the left-most two columns. Only a
V_CM range equal to or less than this will give distortion-
free performance. Conversely, if the maximum V_CM
range is known, the largest acceptable peak value of V_IN
can be found in the corresponding gain column. All
values of V_IN less than this will give full rated perfor-
mance.
Table 4. Input Common-Mode Voltage Range
Limitations
Input Voltage Magnitude (Volts-Peak)
V_CM-
1.000
V_CM+
4.000
G=1 G=2 G=5 G=10
0.557 0.278 0.111 0.056
1.100
3.900 0.728 0.364 0.146 0.073
1.200
3.800 0.899 0.450 0.180 0.090
1.300
3.700 1.071 0.535 0.214 0.107
1.400
3.600 1.242 0.621 0.248 0.124
1.500
3.500 1.413 0.707 0.283 0.141
1.600
3.400 1.584 0.792 0.317 0.158
1.700
3.300 1.756 0.878 0.351 0.176
1.800
3.200 1.927 0.964 0.385 0.193
1.900
3.100 2.098 1.049 0.420 0.210
2.000
3.000 2.270 1.135 0.454 0.227
2.100
2.900 2.441 1.220 0.488 0.244
2.200
2.800 2.612 1.306 0.522 0.261
2.300
2.700 2.783 1.392 0.557 0.278
2.400
2.600 2.955 1.477 0.591 0.295
2.426
2.574 3.000* 1.500* 0.600* 0.300*
2.500
2.500 3.126 1.563 0.625 0.313
*Peak input voltage for guaranteed performance at a given gain setting.
11