THS5661 Datasheet, PDF (17/29 Pages) Texas Instruments – 12-BIT, 100 MSPS, CommsDACE DIGITAL-TO-ANALOG CONVERTER

THS5661
12-BIT, 100 MSPS, CommsDAC™
DIGITAL-TO-ANALOG CONVERTER
SLAS200A – MAY 1999 – REVISED JULY 1999
APPLICATION INFORMATION
reference operation
The THS5661 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_BIAS. The bias current I_BIAS through resistor
R_BIAS is defined by the on-chip bandgap reference voltage and control amplifier. The full-scale output current
equals 32 times this bias current. The full-scale output current IOUT_FS can thus be expressed as:
+ + IOUT_FS
32
I_BIAS
32 V_EXTIO
R_BIAS
where V_EXTIO is the voltage at terminal EXTIO. The bandgap reference voltage delivers an accurate voltage
of 1.2 V. This reference is active when terminal EXTLO is connected to AGND. An external decoupling capacitor
C_EXT of 0.1 µF should be connected externally to terminal EXTIO for compensation. The bandgap reference
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_DD. Capacitor C_EXT may
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_BIAS or changing
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
–
THS4041
+
External
Control AMP
AVDD
EXTLO
1.2 V
REF
AGND
SLEEP
1 nF
COMP1
AVDD
AVDD
Current Source Array
EXTIO
BIASJ
REXT
–
REF AMP
+
Internal
Control AMP
IOUT1 or IOUT2
Figure 25. Bypassing the Internal Reference and Control Amplifier
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