Datasheet AD9280 (Analog Devices) - 10

AD9280 SUMMARY OF MODES VOLTAGE REFERENCE 1 V Mode
the internal reference may be set to 1 V by connect-
AIN A/D SHA REFTS CORE
ing REFSENSE and VREF together.
2 V Mode
the internal reference my be set to 2 V by connecting REFSENSE to analog ground
AD9280 External Divider Mode
the internal reference may be set to a point between 1 V and 2 V by adding external resistors. See
REFBS
Figure 16f.
External Reference Mode
enables the user to apply an exter- Figure 15. AD9280 Equivalent Functional Input Circuit nal reference to REFTS, REFBS and VREF pins. This mode In single-ended operation, the input spans the range, is attained by tying REFSENSE to VDD. REFBS ≤ AIN ≤ REFTS
REFERENCE BUFFER
where REFBS can be connected to GND and REFTS con-
Center Span Mode
midscale is set by shorting REFTS and nected to VREF. If the user requires a different reference range, REFBS together and applying the midscale voltage to that point REFBS and REFTS can be driven to any voltage within the The MODE pin is set to AVDD/2. The analog input will swing power supply rails, so long as the difference between the two is about that midscale point. between 1 V and 2 V.
Top/Bottom Mode
sets the input range between two points. In differential operation, REFTS and REFBS are shorted to- The two points are between 1 V and 2 V apart. The Top/Bottom gether, and the input span is set by VREF, Mode is enabled by tying the MODE pin to AVDD. (REFTS – VREF/2) ≤ AIN ≤ (REFTS + VREF/2) where VREF is determined by the internal reference or brought
ANALOG INPUT
in externally by the user.
Differential Mode
is attained by driving the AIN pin as one differential input, shorting REFTS and REFBS together and The best noise performance may be obtained by operating the driving them as the second differential input. The MODE pin AD9280 with a 2 V input range. The best distortion perfor- is tied to AVDD/2. Preferred mode for optimal distortion mance may be obtained by operating the AD9280 with a 1 V performance. input range.
Single-Ended
is attained by driving the AIN pin while the
REFERENCE OPERATION
REFTS and REFBS pins are held at dc points. The MODE pin is The AD9280 can be configured in a variety of reference topolo- tied to AVDD. gies. The simplest configuration is to use the AD9280’s onboard
Single-Ended/Clamped (AC Coupled)
the input may be bandgap reference, which provides a pin-strappable option to clamped to some dc level by ac coupling the input. This is done generate either a 1 V or 2 V output. If the user desires a refer- by tying the CLAMPIN to some dc point and applying a pulse ence voltage other than those two, an external resistor divider to the CLAMP pin. MODE pin is tied to AVDD. can be connected between VREF, REFSENSE and analog ground to generate a potential anywhere between 1 V and 2 V.
SPECIAL
Another alternative is to use an external reference for designs
AD876-8 Mode
enables users of the AD876-8 to drop the requiring enhanced accuracy and/or drift performance. A AD9280 into their socket. This mode is attained by floating or third alternative is to bring in top and bottom references, grounding the MODE pin. bypassing VREF altogether. Figures 16d, 16e and 16f illustrate the reference and input ar-
INPUT AND REFERENCE OVERVIEW
chitecture of the AD9280. In tailoring a desired arrangement, Figure 16, a simplified model of the AD9280, highlights the the user can select an input configuration to match drive circuit. relationship between the analog input, AIN, and the reference Then, moving to the reference modes at the bottom of the voltages, REFTS, REFBS and VREF. Like the voltages applied figure, select a reference circuit to accommodate the offset and to the resistor ladder in a flash A/D converter, REFTS and amplitude of a full-scale signal. REFBS define the maximum and minimum input voltages to the A/D. Table I outlines pin configurations to match user requirements. The input stage is normally configured for single-ended opera- tion, but allows for differential operation by shorting REFTS and REFBS together to be used as the second input. REV. E –9– Document Outline FEATURES PRODUCT DESCRIPTION PRODUCT HIGHLIGHTS FUNCTIONAL BLOCK DIAGRAM AD9280-SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ESD CAUTION PIN CONFIGURATION PIN FUNCTION DESCRIPTIONS DEFINITIONS OF SPECIFICATIONS TYPICAL CHARACTERIZATION CURVES APPLYING THE AD9280 Theory of Operation Operational Modes Summary of Modes Voltage Reference Reference Buffer Analog Input Special Input and Reference Overview REFERENCE OPERATION Internal Reference Operation External Reference Operation STANDBY OPERATION CLAMP OPERATION Clamp Circuit Example DRIVING THE ANALOG INPUT DIFFERENTIAL INPUT OPERATION AD876-8 MODE OF OPERATION CLOCK INPUT DIGITAL INPUTS AND OUTPUTS APPLICATIONS Direct IF Down Conversion Using the AD9280 Grounding and Layout Rules Digital Outputs Three-State Outputs OUTLINE DIMENSIONS Ordering Guide REVISION HISTORY