ADuCM320 Hardware Reference Manual
UG-498
Rev. C | Page 43 of 196
VDACs
VDAC FEATURES
has eight VDACs. The specified load resistance is greater than 5 kΩ, and the specified capacitance is less than 100 pF.
VDAC BLOCK DIAGRAM
V
REF
STRING DAC
DACOUT
DAC_BUF
11176
-010
Figure 11. Output Mode Capacitor Load ≤ 100 pF
VDAC OVERVIEW
has eight VDACs specified to drive 5 kΩ load, 500 μA maximum.
The VDACs can select from two reference sources:
0 V to internal reference, V
REF
(0 V to 2.5 V)
0 V to AV
DD
(3.3 V)
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.
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; in 0-to-AV
DD
mode, the linearity specification is also not guaranteed for Code 3995 to Code 4095.
Linearity degradation near ground and AV
DD
is caused by saturation of the output amplifier; a general representation of its effects
(neglecting offset and gain error) is shown in Figure 12.
The dotted line in Figure 12 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 12 represents a transfer function in 0-to-AV
DD
mode only. In 0-
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.
AVDD
0x00000000
0x0FFF0000
1
1176-
016
Figure 12. DAC Endpoint Nonlinearities Due to Amplifier Saturation