Idacs; Idac Features; Idac Block Diagram; Idac Overview - Analog Devices ADuCM320 Hardware Reference Manual

ADuCM320 Hardware Reference Manual

IDACs

IDAC FEATURES

The ADuCM320 provides four IDACs. These are low noise, low drift current source outputs.

IDAC0, IDAC1, IDAC2 and IDAC3: 0 mA to 150 mA full-scale output, bias current setting for optical laser.

IDAC BLOCK DIAGRAM

3.3V
2.5V OUT
LDO
1.2V
VREF
BUF
3.16kΩ

IDAC OVERVIEW

Precision Current Generation and Fault Protection
The reference current for the IDACs is generated by a precision internal band gap voltage reference (V
resistor (R , 5 ppm, 0.1%). The reference current is equal to V
REF
voltage source that helps to minimize the overall IDAC gain error and gain error drift. The noise of the IDAC outputs is limited by the
low-pass filter on the output stage; each IDAC requires a 10 nF capacitor between PVDD and its CDAMP pin.
Figure 9 shows the typical architecture of the IDAC. The parallel 11-bit and 5-bit IDACs set the output current. The output of these
IDACs are summed together and fed to a current mirror and then are gained up at the output stage.
Production trimming of the LDO band gap reference aids performance. In addition, gain trimming and scaling of the current mirror and
output stages are also included in the ATE test program.
IDAC Shutdown
IDAC0, IDAC1, IDAC2, and IDAC3 also have a small current sink capability to minimize the offset current when the data register is set
to 0. The IDACxCON[1] bit can be used to enable a pull-down current source to PGND. This pull-down current is typically 100 μA.
IDAC Output Filter
Each IDAC has a filter on the output stage to minimize noise. Each IDAC requires an external 10 nF capacitor between PVDD and its
CDAMP pin as per the ADuCM320
Table 26. IDAC Filter Bandwidth Control Settings
IDACxCON[5:2]
0000
0101
0110
0111
1000
1001
All other options are reserved
AGND
11-BIT 5-BIT
IDAC IDAC
R
REF
5ppm
0.1% PGND
Figure 9. Example IDAC Circuit—IDAC3
data sheet. The on-chip, programmable resistor is controlled by the IDACxCON[5:2] bits.
R Value
60 Ω
5.6 kΩ
11.2 kΩ
22.2 kΩ
44.4 kΩ
104 kΩ
PVDD
CDAMP
10nF
BUF
PGND
ADC
INPUT
MUX
PGND
÷ R . The band gap voltage reference is a low drift, high accuracy
BANDGAP REF
Cutoff Frequency (f
262 kHz
2.8 kHz
1.4 kHz
715 Hz
357 Hz
153 Hz
Rev. C | Page 37 of 196
PVDD
PULL-DOWN
100µA
SINK
CURRENT
PGND
)) and an external precision
BANDGAP
)
C
UG-498
IDAC3