AD7865BS-1 Analog Devices Inc, AD7865BS-1 Datasheet - Page 9

• Manufacturer Part Number: AD7865BS-1
• Description: IC ADC 14BIT 4CH 5V 44-MQFP
• Manufacturer: Analog Devices Inc

Specifications of AD7865BS-1

• Rohs Status: RoHS non-compliant
• Number Of Bits: 14
• Sampling Rate (per Second): 350k
• Data Interface: Parallel
• Number Of Converters: 1
• Power Dissipation (max): 160mW
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 44-MQFP, 44-PQFP

CONVERTER DETAILS
The AD7865 is a high speed, low power, four-channel simulta-
neous sampling 14-bit A/D converter that operates from a single
+5 V supply. The part contains a 2.4 s successive approxima-
tion ADC, four track/hold amplifiers, an internal +2.5 V refer-
ence and a high speed parallel interface. There are four analog
inputs which can be sampled simultaneously, thus preserving
the relative phase information of the signals on all four analog
inputs. Thereafter, conversions will be completed on the se-
lected subset of the four channels. The part accepts an analog
input range of 10 V or 5 V (AD7865-1), 0 V to +2.5 V or
0 V to +5 V (AD7865-2) and 2.5 V (AD7865-3). Overvoltage
protection on the analog inputs for the part allows the input
voltage to go to 18 V (AD7865-1 with 10 V input range), 9 V
(AD7865-1 with 5 V input range), –1 V to +18 V (AD7865-2)
and –4 V to +18 V (AD7865-3) without causing damage or effect-
ing the conversion result of another channel. The AD7865 has
two operating modes Reading Between Conversions and Reading
after the Conversion Sequence. These modes are discussed in more
detail in the Timing and Control section.
A conversion is initiated on the AD7865 by pulsing the CONVST
input. On the rising edge of CONVST, all four on-chip track/
holds are simultaneously placed into hold and the conversion
sequence is started on all the selected channels. Channel selec-
tion is made via the SL1–SL4 pins if H/S SEL is logic zero, or
via the channel select register if H/S SEL is logic one—see
Selecting a Conversion Sequence. The channel select register is
programmed via the bidirectional data lines DB0–DB3 and a
standard write operation. The selected conversion sequence is
latched on the rising edge of CONVST so changing a selection
will only take effect once a new conversion sequence is initiated.
The BUSY output signal is triggered high on the rising edge of
CONVST and will remain high for the duration of the conver-
sion sequence. The conversion clock for the part is generated
internally using a laser-trimmed clock oscillator circuit. There is
also the option of using an external clock, by tying the INT/
EXT CLK pin logic high and applying an external clock to the
CLKIN pin. However, the optimum throughput is obtained by
using the internally generated clock— see Using an External
Clock. The EOC signal indicates the end of each conversion in the
conversion sequence. The BUSY signal indicates the end of the
full conversion sequence and at this time all four Track and Holds
return to tracking mode. The conversion results can either be read
at the end of the full conversion sequence (indicated by BUSY
going low) or as each result becomes available (indicated by EOC
going low). Data is read from the part via a 14-bit parallel data bus
with standard CS and RD signals—see Timing and Control.
Conversion time for each channel of the AD7865 is 2.4 s and
the track/hold acquisition time is 0.35 s. To obtain optimum
performance from the part, the read operation should not occur
during a channel conversion or during the 100 ns prior to the
next CONVST rising edge. This allows the part to operate at
throughput rates up to 100 kHz for all four channels and achieve
data sheet specifications.
Track/Hold Section
The track/hold amplifiers on the AD7865 allows the ADCs to
accurately convert an input sine wave of full-scale amplitude to
14-bit accuracy. The input bandwidth of the track/hold is greater
than the Nyquist rate of the ADC even when the ADC is oper-
ated at its maximum throughput rate of 350 kSPS (i.e., the
track/hold can handle input frequencies in excess of 175 kHz).
REV. A
–9–
The track/hold amplifiers acquire input signals to 14-bit accu-
racy in less than 350 ns. The operation of the track/holds are
essentially transparent to the user. The four track/hold amplifi-
ers sample their respective input channels simultaneously, on
the rising edge of CONVST. The aperture time for the track/
holds (i.e., the delay time between the external CONVST signal
and the track/hold actually going into hold) are typically 15 ns
and, more importantly, is well matched across the four track/
holds on one device and also well matched from device to de-
vice. This allows the relative phase information between differ-
ent input channels to be accurately preserved. It also allows
multiple AD7865s to sample more than four channels simulta-
neously. At the end of a conversion sequence, the part returns
to its tracking mode. The acquisition time of the track/hold
amplifiers begins at this point.
The autozero section of the track/hold circuit is designed to
work with input slew rates of up to 4
This corresponds to a full-scale sine wave of up to 4 MHz for
any input range. Slew rates above this level within the acquisi-
tion time may cause an incorrect conversion result to be re-
turned from the AD7865.
Reference Section
The AD7865 contains a single reference pin, labelled V
which either provides access to the part’s own +2.5 V reference
or allows an external +2.5 V reference to be connected to pro-
vide the reference source for the part. The part is specified with
a +2.5 V reference voltage.
The AD7865 contains an on-chip +2.5 V reference. To use this
reference as the reference source for the AD7865, simply con-
nect a 0.1 F disc ceramic capacitor from the V
The voltage that appears at this pin is internally buffered before
being applied to the ADC. If this reference is required for use
external to the AD7865, it should be buffered as the part has a
FET switch in series with the reference output, resulting in a
source impedance for this output of 6 k nominal. The toler-
ance on the internal reference is 10 mV at +25 C with a typi-
cal temperature coefficient of 25 ppm/ C and a maximum error
over temperature of 20 mV.
If the application requires a reference with a tighter tolerance or
the AD7865 needs to be used with a system reference, the user
has the option of connecting an external reference to this V
pin. The external reference will effectively overdrive the internal
reference and thus provide the reference source for the ADC.
The reference input is buffered before being applied to the ADC
with the maximum input current of 100 A. Suitable reference
sources for the AD7865 include the AD680, AD780, REF192
and REF43 precision +2.5 V references.
CIRCUIT DESCRIPTION
Analog Input Section
The AD7865 is offered as three part types, the AD7865-1 where
each input can be configured for 10 V or a 5 V input voltage
range, the AD7865-3 which handles input voltage range 2.5 V
and the AD7865-2 which has an input voltage range of 0 V to
+2.5 V or 0 V to +5 V. The amount of current flowing into the
analog input will depend on the analog input range and the analog
input voltage. The maximum current flows when negative full
scale is applied.
(Full-Scale Span).
REF
AD7865
pin to AGND.
REF
,
REF