ISPPAC30-01SI LATTICE [Lattice Semiconductor], ISPPAC30-01SI Datasheet - Page 16

• Manufacturer Part Number: ISPPAC30-01SI
• Description: In-System Programmable Analog Circuit
• Manufacturer: LATTICE [Lattice Semiconductor]

Lattice Semiconductor
ispPAC30 Preliminary Data Sheet
In this example, the effective resolution provided by combining the two references would normally require a 13-bit
DAC to replicate. Keep in mind, however, that resolution is not the same as accuracy. The absolute accuracy pro-
vided by an ispPAC30 using this technique is approximately equivalent to that provided by a 10-bit DAC.
In many situations, such as those in which a parameter is being interactively adjusted for optimal performance,
absolute accuracy may not be of paramount importance. In this case, stability and resolution of the adjustment are
more important than the absolute accuracy of the adjustment.
Interfacing to ispPAC Inputs
As mentioned in the previous IA section, any input voltage between 0 to 2.8V can be applied directly to an
ispPAC30 input. To keep the output from trying to swing below 0V, if Vin- is more positive than Vin+, an offsetting
signal must be applied to the appropriate summing node to balance or counteract the negative input. Single-ended
connections, however, only require that the minus input be connected to 0V or some other fixed voltage. More infor-
mation on inputting signals to ispPAC30 can be found in application note AN6026, Interfacing to ispPAC Differential
Inputs.
Although differential signaling offers many significant benefits in a design, most analog designs today still use sin-
gle-ended signals where system ‘ground’ is used as a global zero-volt reference. The differential inputs provided on
ispPAC products provide more than enough flexibility to accommodate single-ended signals.
Figure 2. DC Coupling a Single-ended Signal
ispPAC30
IA
0V to +2.8V
input
When using an ispPAC30 with a single-ended input (Figure 2), tie the unused terminal to a reference voltage. Since
the common-mode input range for the ispPAC30 includes ground, the minus input is most often connected there.
This results in an internal signal value which corresponds directly to the input signal voltage (e.g. a +1.67V input
results in +1.67V of signal internally). When using an ispPAC30 in this manner, it will accommodate single-ended
input signals ranging from 0V to +2.8V.
In systems operating from single +5V supplies, it is often desirable to be able to accommodate rail-to-rail signals,
which range from ground to the positive supply voltage (+5V). Figure 3 shows an interface circuits that allows
ispPAC30 inputs to accept 0-5V signals, where R1 and R2 divide down the signal input.
Figure 3. Interfacing to a 0-5V DC Signal
R1
ispPAC30
100kΩ
IA
0V to +5V
Input
R2
100kΩ
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