Actel Fusion Mixed-Signal FPGAs
The Analog Quad offers a wide variety of prescaling options to enable the ADC to resolve the input
signals. Figure 2-67 shows the path through the Analog Quad for a signal that is to be prescaled
prior to conversion. The ADC internal reference voltage and the prescaler factors were selected to
make both prescaling and postscaling of the signals easy binary calculations (refer to Table 2-54 on
page 2-128 for details). When an analog input pad is configured with a prescaler, there will be a
1 MΩ resistor to ground. This occurs even when the device is in power-down mode. In low power
standby or sleep mode (VCC is OFF, VCC33A is ON, VCCI is ON) or when the resource is not used,
analog inputs are pulled down to ground through a 1 MΩ resistor. The gate driver output is
floating (or tristated), and there is no extra current on VCC33A
.
These scaling factors hold true whether the particular pad is configured to accept a positive or
negative voltage. Note that whereas the AV and AC pads support the same prescaling factors, the
AT pad supports a reduced set of prescaling factors and supports positive voltages only.
Typical scaling factors are given in Table 2-54 on page 2-128, and the gain error (which contributes
to the minimum and maximum) is in Table 2-46 on page 2-115.
Off-Chip
AV
AC
AG
AT
Voltage
Monitor Block
Current
Monitor Block
Gate
Driver
Temperature
Monitor Block
Pads
On-Chip
Analog Quad
Prescaler
Prescaler
Prescaler
Power
MOSFET
Gate Driver
Digital
Input
Digital
Input
Digital
Input
Current
Monitor / Instr
Amplifier
Temperature
Monitor
To FPGA
(DAVOUTx)
To FPGA
(DACOUTx)
From FPGA
(GDONx)
To FPGA
(DATOUTx)
To Analog MUX
To Analog MUX
To Analog MUX
Figure 2-67 • Analog Quad Prescaler Input Configuration
Preliminary v1.7
2-87
ACTEL [ Actel Corporation ]
