ADA4841-X AD [Analog Devices], ADA4841-X Datasheet - Page 15

• Manufacturer Part Number: ADA4841-X
• Manufacturer: AD [Analog Devices]

Preliminary Technical Data
• For ac applications, the driver should have a THD
• For multichannel, multiplexed applications on each input or
Table 10. Recommended Driver Amplifiers
Amplifier
ADA4841-x
AD8655
AD8021
AD8022
OP184
AD8605,
When the source impedance of the driving circuit is low, the
AD7689 can be driven directly. Large source impedances
significantly affect the ac performance, especially total
harmonic distortion (THD). The dc performances are less
sensitive to the input impedance. The maximum source
impedance depends on the amount of THD that can be
tolerated. The THD degrades as a function of the source
impedance and the maximum input frequency.
VOLTAGE REFERENCE OUTPUT/INPUT
The AD7689 allows the choice of either a very low temperature
drift internal voltage reference, an external reference or an external
buffered reference.
The internal reference of the AD7689 provides excellent perfor-
mance and can be used in almost all applications. There are a
possible 6 choices of voltage reference schemes briefly described
in Table 9 with further details in each of the following sections.
Internal Reference/Temperature Sensor
The internal reference can be set for either 2.5V or a 4.096V
output as detailed in Table 9. With the internal reference enabled,
the band-gap voltage will also be present on the REFIN pin, which
requires an external 0.1 μF capacitor.
Enabling the reference also enables the internal temperature sensor,
which measures the internal temperature of the AD7689 thus
useful for performing a system calibration. Note that when using
performance commensurate with the AD7689. TBD shows
the AD7689’s THD vs. frequency.
input pair, the driver amplifier and the AD7689 analog input
circuit must settle a full-scale step onto the capacitor array at
a 16-bit level (0.0015%). In the amplifier’s data sheet, settling
at 0.1% to 0.01% is more commonly specified. This could
differ significantly from the settling time at a 16-bit level and
should be verified prior to driver selection.
where:
f
(1.7MHz in full BW or 425kHz in ¼ BW) or the cutoff
frequency of an input filter, if one is used.
N is the noise gain of the amplifier (for example, 1 in buffer
configuration).
e
nV/√Hz.
–3dB
N
is the equivalent input noise voltage of the op amp, in
AD8615
is the input bandwidth in MHz of the AD7689
Typical Application
Very low noise, small, and low power
5 V single supply, low noise
Very low noise and high frequency
Low noise and high frequency
Low power, low noise, and low frequency
5 V single supply, low power
Rev. PrC | Page 15 of 20
the temperature sensor, the output is straight binary referenced
from the AD7689 GND pin.
The internal reference is temperature-compensated to within
15 mV. The reference is trimmed to provide a typical drift of
3 ppm/°C. This typical drift characteristic is shown in TBD.
External Reference and Internal Buffer
For improved drift performance, and external reference can be
used with the internal buffer. The external reference is con-
nected to REFIN and the output is produced on the REF pin.
There are two modes which can use en external reference with
the internal buffer; one with the temperature sensor enabled
and one without. Refer to Table 9 for the register details. With the
buffer enabled, the gain us unity and limited to input/output of
4.096V.
The internal reference buffer is useful in multi-converter
applications since a buffer is typically required in these
applications. Also, the use of a low power reference can be used
since the internal buffer provides the necessary performance to
drive the SAR architecture of the AD7689.
External Reference
In any of the six modes, an external reference can be connected
directly on the REF pin since the output impedance of REF is >
5k ohms. To reduce power consumption, the reference and
buffer can be powered down independently or together for the
lowest power consumption. However, for applications requiring
the use of the temperature sensor, the reference needs to be
active. Refer to Table 9 for register details.
For improved drift performance, an external reference such as
the
Reference Decoupling
Whether using an internal or external reference, the AD7689
voltage reference output/input, REF, has a dynamic input
impedance and should therefore be driven by a low impedance
source with efficient decoupling between the REF and GND
pins. This decoupling depends on the choice of the voltage
reference, but usually consists of a low ESR capacitor connected
to REF and GND with minimum parasitic inductance. A 22 μF
(X5R, 1206 size) ceramic chip capacitor is appropriate when
using either the internal reference, the
external reference or from a low impedance buffer such as the
AD8031
The placement of the reference decoupling is also important to
the performance of the AD7689, as explained in the Layout
section. The decoupling capacitor should be mounted on the
same side as the ADC right at the REF pin with a thick PCB
trace. The GND should also connect to the reference
decoupling capacitor with the shortest distance and to the
analog ground plane with several vias.
If desired, smaller reference decoupling capacitor values down
to 2.2 μF can be used with a minimal impact on performance,
especially DNL.
ADR43x
or the AD8605.
or
ADR44x
is recommended.
ADR43x /ADR44x
AD7689