W2
1
2
3
DAC Clock
CLKOUT
DAC
J5
J1-33
Alternate Hardware Configurations
10
SLAU726 – August 2017
Copyright © 2017, Texas Instruments Incorporated
DAC90x Evaluation Module
3.1.3
REF
in
Pin
An external reference voltage is provided via the REF
in
input pin. U7 or U8 is used to generate the
external reference voltage. Jumper W4 is for selecting the external reference voltage.
3.2
External Clock Generation
When connected to an AVDD supply of 5 V and DVDD supply of 5 V, the maximum operating speed of
the DAC90x is 200 MHz. When the DVDD supply is 3.3 V, the maximum clock speed is 165 MHz. The
clock signal comes from either the DSP CLKOUT signal or from an external source via J5. The
DAC90xEVM requires an external clock source when used with the TSW1400EVM.
Figure 7. External Clock Signal
3.3
Analog Output Circuits
The analog signal output of the DAC90x can be configured to drive a resistive load, a transformer, an
operational amplifier, or other output configuration, as long as the limitations set by the output voltage
compliance and full-scale range are not exceeded.
3.3.1
Transformer-Coupled Output
The optimum dynamic performance of the DAC90x is achieved with the outputs used differentially. The
DAC90xEVM is configured to operate with transformer output coupling by default. The 1:1 RF transformer
is used for impedance matching, dc isolation, and interfacing between the differential outputs of the DAC
and an external single-ended device. The resistors R23, R24, and R29 are used to form a network that
reflects 50
Ω
across the primary circuit (J6).
A spectrum analyzer with 50-
Ω
input impedance is normally used to measure the performance of the DAC.
A 50-
Ω
coaxial cable is used to connect J6 output to the 50-
Ω
input of the spectrum analyzer. The
spectrum analyzer impedance is in parallel to the transformer 50-
Ω
reflected impedance across J6.
Therefore, the voltage at the input of the spectrum analyzer is ½ J6. If a larger voltage is required, use a
1:2 step up voltage ratio transformer to produce twice the voltage at J6. In addition set R23 to 200
Ω
and
remove R24 and R29. R14 may require some adjustments.
3.3.2
Direct Output
Direct output configuration allows the user to connect the outputs of the DAC90x directly to external
circuitry. This configuration requires that J8 and J9 be installed by the user. A 1-V output will appear at J8
and J9 when IOUT1 and IOUT2 are terminated into 50-
Ω
external resistor loads (output taken from J8 and
J9 and W6 and W7 open).
3.3.3
Operational Amplifier Output
In addition to the transformer-coupled output configuration, the DAC90xEVM offers three more
configurations which use the THS3001 operational amplifier. The THS3001 can be configured as a
difference amplifier, an inverting amplifier, and a non-inverting amplifier.
