AD7690 datasheet(17/29 Pages) AD

17 / 29 page

AD7903

Data Sheet

Rev. A | Page 16 of 28

DRIVER AMPLIFIER CHOICE

Although the AD7903 is easy to drive, the driver amplifier must

meet the following requirements:

•

The noise generated by the driver amplifier must be kept

as low as possible to preserve the SNR and transition noise

performance of the AD7903. The noise from the driver is

filtered by the one-pole, low-pass filter of the AD7903

analog input circuit, made by RIN and CIN or by the

external filter, if one is used. Because the typical noise of

the AD7903 is 40 µV rms, the SNR degradation due to the

amplifier is













−

)

(

log

3dB

LOSS

SNR

where:

f−3dB is the input bandwidth, in megahertz, of the AD7903

(10 MHz) or the cutoff frequency of the input filter, if one

is used.

N is the noise gain of the amplifier (for example, gain = 1

in buffer configuration; see Figure 35).

eN is the equivalent input noise voltage of the op amp, in

nV/√Hz.

•

For ac applications, the driver must have a THD performance

that is commensurate with the AD7903.

•

For multichannel, multiplexed applications, the driver

amplifier and the AD7903 analog input circuit must settle

for a full-scale step onto the capacitor array at a 16-bit level

(0.0015%, 15 ppm). In the amplifier data sheet, settling at

0.1% to 0.01% is more commonly specified. This may differ

significantly from the settling time at a 16-bit level. Be sure

to verify the settling time prior to driver selection.

Table 8. Recommended Driver Amplifiers

Amplifier

Typical Application

ADA4941-1

Very low noise, low power, single to differential

ADA4841-x

Very low noise, small, and low power

AD8021

Very low noise and high frequency

AD8022

Low noise and high frequency

OP184

Low power, low noise, and low frequency

AD8655

5 V single supply, low noise

AD8605, AD8615

5 V single supply, low power

SINGLE-TO-DIFFERENTIAL DRIVER

For applications using a single-ended analog signal, either bipolar

or unipolar, the ADA4941-1 single-ended-to-differential driver

allows a differential input to the device. The schematic is shown

in Figure 36.

R1 and R2 set the attenuation ratio between the input range and

the ADC range (VREF). R1, R2, and CF are chosen depending on

the desired input resistance, signal bandwidth, antialiasing, and

noise contribution. For example, for the ±10 V range with a 4 kΩ

impedance, R1 = 4 kΩ and R2 = 1 kΩ.

R3 and R4 set the common mode on the INx− input, and R5 and

R6 set the common mode on the INx+ input of the ADC. The

common mode must be close to VREF/2. For example, for the

±10 V range with a single supply, R3 = 8.45 kΩ, R4 = 11.8 kΩ,

R5 = 10.5 kΩ, and R6 = 9.76 kΩ.

Figure 36. Single-Ended-to-Differential Driver Circuit

20Ω

10µF

100nF

+2.5V

+5V REF

+5.2V

–0.2V

±10V,

±5V, ..

REFx

VDDx

GND

INx+

INx–

AD7903

ADCx

2.7nF

ADA4941-1

OUTP

OUTN

REF

100nF

AD7690 Datasheet(PDF) 17 Page - Analog Devices

AD7690 Datasheet(HTML) 17 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 22 23 24 25 26 27 28 29

Datasheet Download

Link URL