Agilent Technologies 93000 SOC Series Training Manual page 602

THS5641A
8-BIT, 100 MSPS, CommsDAC
Appendix C
DIGITAL-TO-ANALOG CONVERTER
SLAS277A –MARCH 2000 – REVISED SEPTEMBER 2002
reference operation
The THS5641A comprises a bandgap reference and control amplifier for biasing the full-scale output current.
The full-scale output current is set by applying an external resistor R
R is defined by the on-chip bandgap reference voltage and control amplifier. The full-scale output current
BIAS
equals 32 times this bias current. The full-scale output current IOUT
+
IOUT 32
FS
where V is the voltage at terminal EXTIO. The bandgap reference voltage delivers an accurate voltage
EXTIO
of 1.2 V. This reference is active when terminal EXTLO is connected to AGND. An external decoupling capacitor
of 0.1 µF should be connected externally to terminal EXTIO for compensation. The bandgap reference
C
EXT
can additionally be used for external reference operation. In that case, an external buffer with high impedance
input should be applied in order to limit the bandgap load current to a maximum of 100 nA. The internal reference
can be disabled and overridden by an external reference by connecting EXTLO to AV
hence be omitted. Terminal EXTIO thus serves as either input or output node.
The full-scale output current can be adjusted from 20 mA down to 2 mA by varying resistor R
the externally applied reference voltage. The internal control amplifier has a wide input range, supporting the
full-scale output current range of 20 dB. The bandwidth of the internal control amplifier is defined by the internal
1 nF compensation capacitor at pin COMP1 and the external compensation capacitor C1. The relatively weak
internal control amplifier may be overridden by an externally applied amplifier with sufficient drive for the internal
1 nF load, as shown in Figure 25. This provides the user with more flexibility and higher bandwidths, which are
specifically attractive for gain control and multiplying DAC applications. Pin SLEEP should be connected to
AGND or left disconnected when an external control amplifier is used.
EXT Reference
Voltage
AVDD
R EXT
Figure 25. Bypassing the Internal Reference and Control Amplifier
16
APPLICATION INFORMATION
+
32 V
EXTIO
I
BIAS
R
BIAS
–
Control AMP
THS4041
+
AGND
SLEEP
1.2 V
1 nF
EXTLO
REF
EXTIO
–
REF AMP
BIASJ
+
Control AMP
602
•
POST OFFICE BOX 655303
. The bias current I
BIAS
can thus be expressed as:
FS
External
AV DD
COMP1 AV DD
Internal
DALLAS, TEXAS 75265
through resistor
BIAS
. Capacitor C may
DD EXT
or changing
BIAS
Current Source Array
IOUT1 or IOUT2