AD7650 datasheet(13/21 Pages) AD

13 / 21 page

REV. 0

AD7650

–12–

• The noise generated by the driver amplifier needs to be kept

as low as possible to preserve the SNR and transition noise

performance of the AD7650. The noise coming from the

driver is filtered by the AD7650 analog input circuit one-pole

low-pass filter made by R1 and C2 or the external filter if any

are used.

• The driver needs to have a THD performance suitable to that

of the AD7650.

The AD8021 meets these requirements and is usually appropri-

ate for almost all applications. The AD8021 needs an external

compensation capacitor of 10 pF. This capacitor should have

good linearity as an NPO ceramic or mica type.

The AD8022 could also be used where dual version is needed

and gain of 1 is used.

The AD829 is another alternative where high-frequency (above

100 kHz) performance is not required. In gain of 1, it requires

an 82 pF compensation capacitor.

The AD8610 is another option where low bias current is needed

in low-frequency applications.

Voltage Reference Input

The AD7650 uses an external 2.5 V voltage reference. The volt-

age reference input REF of the AD7650 has a dynamic input

impedance. Therefore, it should be driven by a low impedance

source with an efficient decoupling between REF and REFGND

inputs. This decoupling depends on the choice of the voltage

reference, but usually consists of a low ESR tantalum capacitor

connected to the REF and REFGND inputs with minimum para-

sitic inductance. 47

µF is an appropriate value for tantalum capacitor

when used with one of the recommended reference voltages:

• The low-noise, low temperature drift ADR421 and AD780

voltage references.

• The low-power ADR291 voltage reference.

• The low-cost AD1582 voltage reference.

For applications using multiple AD7650s, it is more effective to

buffer the reference voltage with a low-noise, very stable op amp

such as the AD8031.

Care should also be taken with the reference temperature coeffi-

cient of the voltage reference which directly affects the full-scale

accuracy if this parameter matters. For instance, a

±15 ppm/°C

tempco of the reference changes the full scale by

±1 LSB/°C.

Note that VREF , as mentioned in the specification table, could

be increased to AVDD –1.85 V. Since the input range is defined

in terms of VREF, this would essentially increase the range to

make it a 0 V to 3 V input range with a reference voltage of 3 V.

The AD780 can be selected with a 3 V reference voltage.

Power Supply

The AD7650 uses three sets of power supply pins: an analog 5 V

supply AVDD, a digital 5 V core supply DVDD, and a digital

input/output interface supply OVDD. The OVDD supply allows

direct interface with any logic working between 2.7 V and 5.25 V.

To reduce the number of supplies needed, the digital core (DVDD)

can be supplied through a simple RC filter from the analog

supply as shown in Figure 5. The AD7650 is independent of

power supply sequencing and thus free from supply voltage

induced latchup.

Analog Input

Figure 6 shows an equivalent circuit of the input structure of the

AD7650.

IN+

OR IN–

AGND

AVDD

Figure 6. Equivalent Analog Input Circuit

The two diodes, D1 and D2, provide ESD protection for the

analog inputs IN+ and IN–. Care must be taken to ensure that the

analog input signal never exceeds the supply rails by more than

0.3 V. This will cause these diodes to become forward-biased and

start conducting current. These diodes can handle a forward-

biased current of 100 mA maximum. For instance, these conditions

could eventually occur when the input buffer’s (U1) supplies are

different from AVDD. In such case, an input buffer with a short

circuit current limitation can be used to protect the part.

This analog input structure allows the sampling of the differential

signal between IN+ and IN–. Unlike other converters, the IN–

input is sampled at the same time as the IN+ input. By using

this differential input, small signals common to both inputs are

rejected. For instance, by using IN– to sense a remote signal

ground, difference of ground potentials between the sensor and

the local ADC ground are eliminated.

During the acquisition phase, the impedance of the analog input

IN+ can be modeled as a parallel combination of capacitor C1

and the network formed by the series connection of R1 and C2.

Capacitor C1 is primarily the pin capacitance. The resistor R1 is

typically 140

Ω and is a lumped component made up of some

serial resistors and the on resistance of the switches. The capacitor

C2 is typically 60 pF and is mainly the ADC sampling capacitor.

During the conversion phase, where the switches are opened,

the input impedance is limited to C1. The R1, C2 makes a one-

pole low-pass filter that reduces undesirable aliasing effect and

limits the noise.

When the source impedance of the driving circuit is low, the

AD7650 can be driven directly. Large source impedances will

significantly affect the ac performances, especially the total

harmonic distortion.

Driver Amplifier Choice

Although the AD7650 is easy to drive, the driver amplifier needs

to meet at least the following requirements:

• The driver amplifier and the AD7650 analog input circuit

must be able together to settle for a full-scale step the capacitor

array at a 16-bit level (0.0015%). In the amplifier’s data sheet,

the settling at 0.1% to 0.01% is more commonly specified. It

could significantly differ from the settling time at 16-bit level

and it should therefore be verified prior to the driver selection.

The tiny op amp AD8021, which combines ultralow noise and

a high-gain bandwidth, meets this settling time requirement

even when used with high gain up to 13.

AD7650 Datasheet(PDF) 13 Page - Analog Devices

AD7650 Datasheet(HTML) 13 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Datasheet Download

Link URL