Download Binary-Weighted Current Mode Digital-to

Binary-Weighted Current Mode Digital-to-Analog Converter
Current Mode D/A converter is a frequently used component in sensor circuit. Many biosensors use such device to generate stimulus to body tissues or nerves. Such devices
normally are required to generate different amount of current upon certain control signals.
Binary-weighted unit control is a commonly used method. The r-th bit of the control
signal controls 2r-1 units so that n control bits can make up any number of active units
from 0 to 2n-1. Therefore, if each unit is a 10µA current source and 4 control bits are
available, any output current from 0A to 150µA with a 10µA step can be achieved.
Unit Current Source
VDD
EN
D
B
Vbias2
G
S
D
G
D
B
S
B
G
S
D
B
G
Iout
S
D
Vbias1
G
B
S
VSS
Figure 1: Unit Current Source
PMOS W/L=3µ/1.2µ
NMOS W/L=1.8µ/1.2µ
VDD=2.5V
VSS=-2.5V
The unit current source can be seen as a regulated cascade current source with a PMOS
switch. The two bias voltages control the output current amount and the EN control
enables or disables the current source output.
Bias Voltage Generator
The bias voltage Vbias1 and Vbias2 are generated through a circuit as shown in figure 2.
The two resistors are external resistors. Therefore, in actual design, pin Rin1 and Rin2
should be used instead of the resistors.
Figure 2: Bias Voltage Generator
D/A Converter
The D/A converter is done in the way that:
i)
Digital input bit 0 to n-1 are connected to the enable signal of 20, 21, …, 2n-1
current sources.
ii)
Only one bias generator is created for all current sources.
iii)
All outputs of the current sources are tied together as the total current output.
The schematic of the complete system is not given, but should be easily conducted from
the description.
Simulation
The simulation of the system uses a testbench schematic as shown in figure 3. A 4-bit
DAC is used as an example.
Figure 3: TestBench of a 4-bit DAC
The four vpulse voltage sources for the D0-D3 inputs should have such characteristics:
i)
ii)
iii)
All 4 pulse voltages should have a 50% duty cycle and Voltage 1 = 5V,
Voltage 2 = 0V
The period of the vpulse connected to Dn should be twice as much as the one
that is connected to Dn-1.
The period of vpulse connected to D0 should be large enough (≥20ns
suggested) so a stable current can be observed during a half cycle.
With such D0 to D3 combinations, during a transient analysis with time t equal to the
period of the vpulse source connected to D3. All possible digital combination from 0000
to 1111 can be obtained.
The output waveform of Iout should show a step current response from 0A to 150µA in a
step of 10µA.