AD9070

–10–

REV. B

Figure 19 shows typical connections for the analog inputs when

using the AD9070 in a dc-coupled system with single-ended

signals. The AD820 is used to offset the ground referenced

input signal to the level required by the AD9070. A very high

performance amplifier, such as the AD9631, is required to avoid

degrading the analog signal presented to the ADC. A buffered

ac interface is easily implemented, with even fewer components

(Figure 20).

AD9070

–5V

GND

1k

0.5V

AIN

AIN

VREF OUT

VREF IN

–5V

–5V

+5V

AD9631

350

AD820

1k

0.1 F

350

1k

0.1 F

0.1 F

Figure 19. DC-Coupled Input

AD9070

0.1 F

–5V

GND

350

1Vp-p

AIN

AIN

VREF OUT

VREF IN

–5V

+5V

AD9631

350

500

0.1 F

0.1 F

500

0.1 F

Figure 20. AC-Coupled Input

Special care was taken in the design of the analog input section

of the AD9070 to prevent damage and corruption of data when

the input is overdriven. The nominal input range is –1.988 V to

–3.012 V (1.024 V p–p centered at –2.5 V). Out-of-range

comparators detect when the analog input signal is out of this

range and set the OR output signal HIGH. The digital outputs

are locked at plus or minus full scale (1FFh or 200h) for

voltages that are out of range but between –1 V and –5 V. Input

voltages outside of this range may result in invalid codes at the

ADCs output.

When the analog input signal returns to the nominal range, the

out-of-range comparators return the ADC to its active mode

and the device recovers in approximately 3 ns.

The input is protected to one volt outside of the power supply

rails. For nominal power (–5 V and ground), the analog input

will not be damaged with signals ranging from –6.0 V to +1.0 V.

Voltage Reference

A stable and accurate –2.5 V voltage reference is built into the

AD9070 (VREF OUT) in the SOIC (BR) package. In normal

operation, the internal reference is used by strapping Pins 3

and 4 of the AD9070 together. The internal reference can

provide 100

µA of extra drive current that may be used for other

circuits.

Some applications may require greater accuracy, improved

temperature performance or adjustment of the gain of the

AD9070, which cannot be obtained by using the internal

reference. For these applications, an external –2.5 V reference

can be connected to VREF IN, which requires 5

µA of drive

current (Figure 21).

AD9070

–5V

GND

VREF OUT

VREF IN

–5V

1.25k

0.1 F

NC

GND

AD780

Figure 21. Using the AD780 Voltage Reference

The input range can be adjusted by varying the reference

voltage applied to the AD9070. No appreciable degradation in

performance occurs when the reference is adjusted

±4%. The

full-scale range of the ADC tracks reference voltage changes

linearly.

Timing

The performance of the AD9070 is insensitive to the duty cycle

of the clock over a wide range of operating conditions: pulse

width variations of as much as

±20% will cause no degradation

in performance (see Figure 15).

The AD9070 provides latched data outputs, with three pipeline

after the rising edge of the encode command (Figure 1). The

length of the output data lines and loads placed on them should

be minimized to reduce transients within the AD9070; these

transients can detract from the converter’s dynamic performance.

The minimum guaranteed conversion rate of the AD9070 is

40 MSPS. At clock rates below 40 MSPS, dynamic performance

may degrade. The AD9070 will operate in bursts, but the user

must flush the internal pipeline each time the clock restarts.

Valid data will be produced on the fourth rising edge of the

ENCODE signal after the clock is restarted.