ADA4254ACPZ-R7 datasheet(25/59 Pages) AD

25 / 59 page

Data Sheet

ADA4254

Rev. B | Page 25 of 59

ensure that CC is at least one magnitude smaller than CD. The

effect of mismatched CC values is reduced with a larger CD:CC

ratio.

+IN

–IN

–OUT

+OUT

ADA4254

Figure 81. External EMI Filter Improves Noise Rejection

INPUT AMPLIFIER

The ADA4254 input amplifier operates on high voltage power

supplies, VDDH and VSSH. The input amplifiers are

monitored for clipping due to excessive signal swing. If excessive

output swing is detected by an input amplifier (A3 and A7 in

Figure 79), the INPUT_ERR flag trips. If INPUT_ERR is

tripped for more than 200 μs, the gain settings in the

GAIN_MUX register are reset to their default values and the

G_RST flag trips. This setup protects the input amplifiers and the

input resistor network. The gain reset function can be disabled via

the G_RST_DIS bit.

OUTPUT AMPLIFIER

The ADA4254 features a fully differential output amplifier

running from dedicated low voltage supplies, AVDD and

AVSS. Use AVDD and AVSS in a single-supply configuration.

By running the output amplifier on low voltage supplies,

circuitry connected to the output of the ADA4254 is inherently

protected. The common-mode output voltage is set by the

VOCM input voltage. VOCM has a high input impedance and

is not biased internally. VOCM also features a 29 MHz EMI

filter to minimize EMI interference. Typically, VOCM is biased

to midsupply through a voltage divider between AVDD and

AVSS to allow the widest swing on the output. The output

amplifier can be set to three different scaling gains via G4 or

G5: 1 V/V, 1.25 V/V, or 1.375 V/V. On power-up or soft reset,

the output amplifier scaling gain defaults to 1 V/V. The output

amplifier is monitored for clipping due to excessive signal

swing. When the output saturates to either supply, the

OUTPUT_ERR error flag trips.

The differential output stage of the ADA4254 allows the device

to be directly connected to high precision ADCs, such as the

AD7768 and the AD4007. When making such a connection, it is

recommended to use a low-pass filter to minimize noise and

aliasing, as shown in Figure 82. The LTC6363 is configured as a

three-pole, low-pass filter with a cutoff frequency of 40 kHz.

1kΩ

10nF

–OUT

+OUT

ADC_IN1

ADC_IN2

Figure 82. Simple Output Filter Preventing Aliasing and Filters Switching Noise

AD4007

OUT–

OUT+

AVSS

GPIO4/CLKIN

10kΩ

CNV

30.1Ω

3.3nF

–

+IN1

VDD

VOCM

GND

IN+

IN–

AVDD

VSSH

VDDH

ADP7112-1.8V

VIN

VOUT

–

3.3nF

ADA4254

LTC6363

49.9Ω

1.4kΩ

3.3nF

536Ω

324Ω

536Ω

324Ω

536Ω

3.3nF

1.4kΩ

49.9Ω

2.5V

28V

–28V

30.1Ω

3.3nF

–IN1

–IN2

+IN2

Figure 83. LTC6363 Used as a Low-Pass Filter/Driver

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ADA4254ACPZ-R7 数据表(PDF) 25 Page - Analog Devices

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