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AD4000 Datasheet(PDF) 20 Page - Analog Devices

Part # AD4000
Description  Precision, Pseudo Differential, SAR ADCs
Download  36 Pages
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Manufacturer  AD [Analog Devices]
Direct Link  http://www.analog.com
Logo AD - Analog Devices

AD4000 Datasheet(HTML) 20 Page - Analog Devices

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AD4000/AD4004/AD4008
Data Sheet
Rev. C | Page 20 of 36
ANALOG INPUTS
Figure 35 shows an equivalent circuit of the analog input structure,
including the overvoltage clamp of the AD4000/AD4004/AD4008.
CEXT
REXT
VIN
REF
D1
IN+
GND
CLAMP
0V TO 15V
RIN
CIN
D2
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 AD4000/AD4004/AD4008
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
AD4000/AD4004/AD4008 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 overvoltage (OV) clamp flag
bit in the register that can be read back (see Table 14), which is
a sticky bit that must be read to be cleared. The status of the
clamp can also be checked in the status bits using an OV 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, where 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 ADA4805-1.
The RC values shown in Table 10 are chosen for ease of drive
considerations and greater ADC input protection. The
combination 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
See the High-Z Mode section
See the High-Z Mode section
<200
200
180
ADA4805-1
>200
200
120
ADA4897-1
Multiplexed
200
120
ADA4897-1


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