AD7665AST datasheet(15/24 Pages) AD

15 / 24 page

REV. 0

AD7665

–15–

FREQUENCY – kHz

–110

100

–100

–90

–80

–70

1000

R = 50

R = 11

R = 100

Figure 8. THD vs. Analog Input Frequency and Input

Resistance (0 V to 2.5 V Only)

Driver Amplifier Choice

Although the AD7665 is easy to drive, the driver amplifier needs

to meet at least the following requirements:

• The driver amplifier and the AD7665 analog input circuit

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

tor 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 speci-

fied. 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 a high gain up to 13.

•

The noise generated by the driver amplifier needs to be kept

as low as possible in order to preserve the SNR and transi-

tion noise performance of the AD7665. The noise coming

from the driver is first scaled down by the resistive scaler

according to the analog input voltage range used, and is then

filtered by the AD7665 analog input circuit one-pole, low-

pass filter made by (R/2 + R1) and CS. The SNR degradation

due to the amplifier is:

SNR

FSR

LOSS





















784

1000

log

–

where

f–3 dB is the –3 dB input bandwidth of the AD7665 (3.6 MHz)

or the cut-off frequency of the input filter if any used

(0 V to 2.5 V range).

is the noise factor of the amplifier (1 if in buffer

configuration).

is the equivalent input noise voltage of the op amp

in nV/(Hz)

1/2.

FSR

is the full-scale span (i.e., 5 V for

±2.5 V range).

For instance, when using the 0 V to 2.5 V range, a driver like

the AD8021, with an equivalent input noise of 2 nV/

√Hz and

configured as a buffer, thus with a noise gain of 1, the SNR

degrades by only 0.12 dB.

• The driver needs to have a THD performance suitable to

that of the AD7665. TPC 8 gives the THD versus frequency

that the driver should preferably exceed.

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 AD7665 uses an external 2.5 V voltage reference. The

voltage reference input REF of the AD7665 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 tanta-

lum capacitor connected to the REF and REFGND inputs with

minimum parasitic inductance. 47

µF is an appropriate value for

the 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 AD7665s, 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.

Scaler Reference Input (Bipolar Input Ranges)

When using the AD7665 with bipolar input ranges, the connec-

tion diagram in Figure 5 shows a reference buffer amplifier.

This buffer amplifier is required to isolate the REFIN pin from

the signal dependent current in the AIN pin. A high-speed op

amp such as the AD8031 can be used with a single 5 V power

supply without degrading the performance of the AD7665. The

buffer must have good settling characteristics and provide low

total noise within the input bandwidth of the AD7665.

Power Supply

The AD7665 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 AD7665 is inde-

pendent of power supply sequencing and thus free from supply

voltage induced latchup. Additionally, it is very insensitive to

power supply variations over a wide frequency range as shown

in Figure 9.

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