AD7765 datasheet(22/24 Pages) AD

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AD7767

Rev. 0 | Page 22 of 24

VREF+ INPUT SIGNAL

The AD7767/AD7767-1/AD7767-2 VREF + pin is supplied with a

5 V input, which is generated by a low noise voltage reference.

Either the ADR445 or the ADR425 can be used with the AD7767/

AD7767-1/AD7767-2 device. This reference voltage input also

acts as a power supply to the AD7767/AD7767-1/AD7767-2

device.

The output of the low noise voltage reference does not require a

buffer; however, it is important to provide a passive filter network

between the VOUT pin of the voltage reference and the VREF+

input on the ADC. Figure 46 shows a reference signal network

that can be used with both the ADR445 and the ADR425.

The 100 nF capacitor on the output of the ADR445 or ADR425

stabilizes the reference output voltage. The series resistor coupled

with the other capacitive values on the reference acts as a low-

pass filter. Figure 46 shows the optimal reference voltage input

circuit.

VREF+

C39

100nF

C40

100µF

C15

10µF

C38

100nF

100Ω

AD7767/

AD7767-1/

AD7767-2

ADR445

ADR425

VOUT

Figure 46. AD7767/AD7767-1/AD7767-2 Reference Filtering,

ADR445 or ADR425 Circuit Topology

For the capacitor designated C40 in Figure 46, either an

electrolytic or tantalum capacitor can be used. This capacitor

acts as a reservoir of charge. Further decoupling capacitors are

placed as close as possible to the VREF+ pin.

MULTIPLEXING ANALOG INPUT CHANNELS

The AD7767/AD7767-1/AD7767-2 can be used with a multi-

plexer configuration. As per any converter that uses a digital

filtering block, the maximum switching rate, or output data rate

per channel, is a function of the digital filter settling time.

A user multiplexing the analog inputs to a converter that

employs a digital filter must wait the full digital filter settling

time before a valid conversion result is achieved; at this point,

the channel can be switched. After switching the channel, the

full settling time must again be observed before a valid conver-

sion result is available and the input is switched once more.

The AD7767 filter settling time equals 74 divided by the output

data rate in use. The maximum switching frequency in a

multiplexed application is therefore 1/(74/ODR), where the

output data rate (ODR) is a function of the applied MCLK

frequency and the decimation rate employed by the device in

question. For example, applying a 1.024 MHz MCLK frequency

to the AD7767 gives a maximum output data rate of 128 kHz,

which in turn allows a 1.729 kHz multiplexer switching rate.

The AD7767-1 and the AD7767-2 employ digital filters with

longer settling time to achieve greater precision; thus, the

maximum switching frequency for these devices is 864 Hz and

432 Hz, respectively.

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