AD7665 datasheet(15/23 Pages) AD

15 / 23 page

REV.

AD7665

–15–

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 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 a 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

ËÁ

¯˜

784

log

–

where:

f–3 dB is the –3 dB input bandwidth in MHz of the AD7665

(3.6 MHz) or the cutoff 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 11 gives the THD versus frequency

that the driver should preferably exceed.

The AD8021 meets these requirements and is usually appropriate

for almost all applications. The AD8021 needs an external com-

pensation capacitor of 10 pF. This capacitor should have good

linearity as an NPO ceramic or mica type.

The AD8022 could also be used where a dual version is needed

and a gain of 1 is used.

The AD829 is another alternative where high frequency (above

100 kHz) performance is not required. In a 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; it should therefore 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 1 µF ceramic capacitor and a

low ESR tantalum 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

like the AD8031.

Care should also be taken with the reference temperature coeffi-

cient of the voltage reference that 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 tables, could

be increased to AVDD – 1.85 V. The benefit here is the increased

SNR obtained as a result of this increase. Since the input range

is defined in terms of VREF, this would essentially increase the

±REF range from ±2.5 V to ±3 V and so on with an AVDD above

4.85 V. The theoretical improvement as a result of this increase

in reference is 1.58 dB (20 log [3/2.5]). Due to the theoretical

quantization noise, however, the observed improvement is approxi-

mately 1 dB. The AD780 can be selected with a 3 V reference

voltage.

Scaler Reference Input (Bipolar Input Ranges)

When using the AD7665 with bipolar input ranges, the connection

diagram in Figure 5 shows a reference buffer amplifier. This

buffer amplifier is required to isolate the REF pin from the signal

dependent current in the INx 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

5V 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 DVDD

+ 0.3 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 indepen-

dent of power supply sequencing, once OVDD does not exceed

DVDD by more than 0.3 V, and thus free from supply voltage

induced latch-up. Additionally, it is very insensitive to power

supply variations over a wide frequency range as shown in Figure 9.

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