ADuCM310 Hardware Reference Manual
UG-549
Rev. C | Page 47 of 192
VDAC OPERATION
The DAC is configurable through a control register and a data register. The on-chip DAC architecture consists of a resistor string DAC
followed by an output buffer amplifier, as shown in Figure 11 and Figure 12.
VDAC Channel 0, Channel 1, Channel 4, and Channel 5
These four VDAC channels are fully implemented on the low voltage analog die. They are designed to drive a resistive loads of <300 Ω on
VDAC4 and VDAC5 and as low as 75 Ω on VDAC0 and VDAC1.
When sourcing high currents (>1 mA), set DACxCON[10] to 1, which results in a smaller maximum output voltage value.
When driving a large capacitive load (>100 pF), set DACxCON [9] to 1. All four of these VDACs are capable of driving a 10 nF capacitive
load.
If driving a small capacitive load, <100 pF, and the source current from the VDAC is <1 mA, clear DACxCON[10:9] to 0. This results in a
wider output voltage range and reduces the power consumption of the output buffer stages of each of these VDACs. DACxCON[10:9]
control the switches that enable/disable the feedback circuitry on the output buffer shown in Figure 1 and Figure 2.
When DACxCON [10:9] = 11, the output voltage range is smaller. See the
data sheet specifications for more details on the
output voltage range for each VDAC. The data sheet specifications assume DACxCON [10:9] = 11.
The linearity specification of the DAC when driving a 5 kΩ resistive load to ground is guaranteed through the full transfer function except
for Code 0 to Code 100 and, in 0 V to AV
DD
mode only, Code 3995 to Code 4095. Linearity degradation near ground and AV
DD
is caused
by saturation of the output amplifier, and a general representation of its effects (neglecting offset and gain error) is shown in Figure 13.
The dotted line in Figure 13 indicates the ideal transfer function. The solid line represents what the transfer function may look like with
endpoint nonlinearities due to saturation of the output amplifier. Figure 13 represents a transfer function in 0 V to AV
DD
mode only. In
0 V to V
REF
mode, the lower nonlinearity is similar. However, the upper portion of the transfer function follows the ideal line all the way
to the end showing no signs of endpoint linearity errors.
AV
DD
0x00000000
0x0FFF0000
1
1461-
013
Figure 13. DAC Endpoint Nonlinearities Due to Amplifier Saturation