wm8918 Wolfson Microelectronics plc, wm8918 Datasheet - Page 73

• Manufacturer Part Number: wm8918
• Description: Ultra Low Power Dac For Portable Audio Applications
• Manufacturer: Wolfson Microelectronics plc

Production Data
REFERENCE VOLTAGES AND MASTER BIAS
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This section describes the analogue reference voltage and bias current controls. Note that, under the
recommended usage conditions of the WM8918, these features will be configured by scheduling the
default Start-Up and Shutdown sequences as described in the “Control Write Sequencer” section. In
these cases, the user does not need to set these register fields directly.
The analogue circuits in the WM8918 require a mid-rail analogue reference voltage, VMID. This
reference is generated from AVDD via a programmable resistor chain.
VMID is enabled by setting the VMID_ENA register bit. The programmable resistor chain is
configured by VMID_RES [1:0], and can be used to optimise the reference for normal operation, low
power standby or for fast start-up as described in Table 45. For normal operation, the VMID_RES
field should be set to 01.
The VMID_BUF_ENA bit allows the buffered VMID reference to be connected to unused
inputs/outputs.
The analogue circuits in the WM8918 require a bias current. The bias current is enabled by setting
BIAS_ENA. Note that the bias current source requires VMID to be enabled also.
Table 45 Reference Voltages and Master Bias Enable
R5 (05h)
VMID
Control (0)
R4 (04h)
Bias Control
(0)
REGISTER
ADDRESS
BIT
2:1
6
0
0
VMID_BUF_
VMID_RES
VMID_ENA
BIAS_ENA
LABEL
ENA
[1:0]
DEFAULT
00
0
0
0
Enable VMID buffer to unused Inputs/Outputs
0 = Disabled
1 = Enabled
VMID Divider Enable and Select
00 = VMID disabled (for OFF mode)
01 = 2 x 50k divider (for normal operation)
10 = 2 x 250k divider (for low power standby)
11 = 2 x 5k divider (for fast start-up)
Enable VMID master bias current source
0 = Disabled
1 = Enabled
Enables the Normal bias current generator
(for all analogue functions)
0 = Disabled
1 = Enabled
PD, Rev 4.0, September 2010
DESCRIPTION
WM8918
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