AD4002 datasheet(20/36 Pages) AD

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AD4002/AD4006/AD4010

Preliminary Technical Data

Rev. PrA | Page 20 of 36

ANALOG INPUTS

Figure 35 shows an equivalent circuit of the analog input structure,

including the overvoltage clamp of the AD4002/AD4006/AD4010.

CEXT

REXT

VIN

REF

IN+

GND

CLAMP

0V TO 15V

RIN

CIN

CPIN

Figure 35. Equivalent Analog Input Circuit

Input Overvoltage Clamp Circuit

Most ADC analog inputs, IN+ and IN−, have no overvoltage

protection circuitry apart from ESD protection diodes. During

an overvoltage event, an ESD protection diode from an analog

input pin (IN+ or IN−) to REF forward biases and shorts the

input pin to REF, potentially overloading the reference or

causing damage to the device. The AD4002/AD4006/AD4010

internal overvoltage clamp circuit with a larger external resistor

(REXT = 200 Ω) eliminates the need for external protection

diodes and protects the ADC inputs against dc overvoltages.

In applications where the amplifier rails are greater than VREF

and less than ground, it is possible for the output to exceed

the input voltage range of the device. In this case, the AD4002/

AD4006/AD4010 internal voltage clamp circuit ensures that the

voltage on the input pin does not exceed VREF + 0.4 V and prevents

damage to the device by clamping the input voltage in a safe

operating range and avoiding disturbance of the reference,

which is particularly important for systems that share the

reference among multiple ADCs.

If the analog input exceeds the reference voltage by 0.4 V, the

internal clamp circuit turns on and the current flows through

the clamp into ground, preventing the input from rising further

and potentially causing damage to the device. The clamp turns

on before D1 (see Figure 35) and can sink up to 50 mA of current.

When the clamp is active, it sets the OV clamp flag bit in the

that must be read to be cleared. The status of the clamp can also

be checked in the status bits using an overvoltage clamp flag (see

Table 15). The clamp circuit does not dissipate static power in the

off state. Note that the clamp cannot sustain the overvoltage

condition for an indefinite amount of time.

The external RC filter is usually present at the ADC input

to band limit the input signal. During an overvoltage event,

excessive voltage is dropped across REXT, and REXT becomes part

of a protection circuit. The REXT value can vary from 200 Ω to

20 kΩ for 15 V protection. The CEXT value can be as low as 100 pF

for correct operation of the clamp. See Table 1 for input overvoltage

clamp specifications.

The analog input structure allows the sampling of the true

differential signal between IN+ and IN−. By using these

differential inputs, signals common to both inputs are rejected.

By using IN− to sense a remote signal ground, ground potential

differences between the sensor and the local ADC ground are

eliminated.

Switched Capacitor Input

During the acquisition phase, the impedance of the analog

inputs (IN+ or IN−) can be modeled as a parallel combination

of Capacitor CPIN and the network formed by the series connection

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

400 Ω and is a lumped component composed of serial resistors

and the on resistance of the switches. CIN is typically 40 pF and

is mainly the ADC sampling capacitor.

During the conversion phase, in which the switches are open, the

input impedance is limited to CPIN. RIN and CIN make a single-

pole, low-pass filter that reduces undesirable aliasing effects and

limits noise.

RC Filter Values

The RC filter value (represented by R and C in Figure 33 and

Figure 34) and driving amplifier can be selected depending on

the input signal bandwidth of interest at the full throughput.

Lower input signal bandwidth means that the RC cutoff can be

lower, thereby reducing noise into the converter. For optimum

performance at various throughputs, use the recommended RC

values (200 Ω, 180 pF) and the ADA4807-1.

The RC values shown in Table 10 are chosen for ease of drive

considerations and greater ADC input protection. The combi-

nation of a large R value (200 Ω) and small C value results in a

reduced dynamic load for the amplifier to drive. The smaller value

of C means fewer stability and phase margin concerns with the

amplifier. The large value of R limits the current into the ADC

input when the amplifier output exceeds the ADC input range.

Table 10. RC Filter and Amplifier Selection for Various Input Bandwidths

Input Signal Bandwidth (kHz)

R (Ω)

C (pF)

Recommended Amplifier

<10

See the High-Z Mode section

<200

200

180

ADA4807-1

>200

200

120

ADA4897-1

Multiplexed

200

120

ADA4897-1

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