AD7949 datasheet(14/20 Pages) AD

14 / 20 page

AD7689

Preliminary Technical Data

Rev. PrC | Page 14 of 20

ground sense; CFG[12:10] = 10X2.

In this configuration, the IN+ is identified by the channel

in CFG[9:7]. Example: for IN0 = IN1+ and IN1 = IN1-,

CFG[9:7] = 0002; for IN1 = IN1+ and IN0 = IN1-,

CFG[9:7] = 0012

• Figure 14D, sows the inputs configured in any of the above

combinations as the AD7689 can be configured

dynamically.

Sequencer

The AD7689 includes a channel sequencer useful for scanning

channels in a IN0 to INn fashion. Channels are scanned as

single or pairs and with or without the temperature sensor, after

the last channel is sequenced.

The sequencer starts with IN0 and finishes with INn set in

CFG[9:7]. For paired channels, the channels are paired

depending on the last channel set in CFG[9:7]. Note that the

channel pairs are always paired IN(even) = INx+ and IN(odd) =

INx- regardless of CFG[7].

To enable the sequencer, CFG[2:1] are written to for initializing

the sequencer. After CFG[13:0] is updated, DIN must be held

low while reading data out (at least for bit 13) or the CFG will

begin updating again.

While operating in a sequence, the CFG can be changed by

writing 012 to CFG[2:1]. However, if changing CFG[11] (paired

or single channel) or CFG[9:7] (last channel in sequence), the

sequence will reinitialize and convert IN0 (or IN1) after CFG is

updated.

Examples

(only bits for input and sequencer are highlighted)

Scan all IN[7:0] referenced to COM = GND sense with

temperature sensor:

CFG

INCC

INn

REF

SEQ

Scan 3 paired channels without temperature sensor and

referenced to VREF/2:

CFG

INCC

INn

REF

SEQ

Scan 4 paired channels referenced to a GND sense with

temperature sensor:

CFG

INCC

INn

REF

SEQ

Input Structure

Figure 15 shows an equivalent circuit of the input structure of

the AD7689.

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

analog inputs, IN[7:0] and COM. Care must be taken to ensure

that the analog input signal does not exceed the supply rails by

more than 0.3 V because this causes the diodes to become

forward biased and to start conducting current. These diodes

can handle a forward-biased current of 130 mA maximum. For

instance, these conditions could eventually occur when the

input buffer’s supplies are different from VDD. In such a case,

for example, an input buffer with a short circuit, the current

limitation can be used to protect the part.

CIN

RIN

CPIN

IN+

OR IN-

OR COM

GND

VDD

Figure 15. Equivalent Analog Input Circuit

The analog input structure allows the sampling of the true

differential signal between INn+ and COM or INn+ and INn-.

By using these differential inputs, signals common to both

inputs are rejected.

During the acquisition phase, the impedance of the analog

inputs can be modeled as a parallel combination of the

capacitor, CPIN, and the network formed by the series

connection of RIN and CIN. CPIN is primarily the pin capacitance.

RIN is typically 3.5kΩ and is a lumped component made up of

serial resistors and the on resistance of the switches. CIN is

typically 27 pF and is mainly the ADC sampling capacitor.

Selectable Low Pass Filter

During the conversion phase, where the switches are opened,

the input impedance is limited to CPIN. While the AD7689 is

acquiring, RIN and CIN make a 1-pole, low-pass filter that reduces

undesirable aliasing effects and limits the noise from the driving

circuitry. The low pass filter can be programmed for the full

bandwidth or ¼ of the bandwidth with CFG[6] as shown in

Table 9.

DRIVER AMPLIFIER CHOICE

Although the AD7689 is easy to drive, the driver amplifier

needs to meet the following requirements:

• 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 AD7689. Note that the AD7689 has a

noise much lower than most of the other 16-bit ADCs and,

therefore, can be driven by a noisier amplifier to meet a given

system noise specification. The noise coming from the

amplifier is filtered by the AD7689 analog input circuit low-

pass filter made by RIN and CIN or by an external filter, if one

is used. Because the typical noise of the AD7689 is 35 μV rms

(with VREF = 5V), the SNR degradation due to the amplifier is

⎟

⎠

⎞

⎜

⎝

⎛

−

3dB

)

(

20log

LOSS

SNR

AD7949 Datasheet(PDF) 14 Page - Analog Devices

AD7949 Datasheet(HTML) 14 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

Datasheet Download

Link URL