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PDF AD7785 Data sheet ( Hoja de datos )
| Número de pieza | AD7785 | |
| Descripción | 20-Bit ADC | |
| Fabricantes | Analog Devices | |
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3-Channel, Low Noise, Low Power, 20-Bit ∑-Δ
ADC with On-Chip In-Amp and Reference
AD7785
FEATURES
Up to 20 bits effective resolution
RMS noise
40 nV @ 4.17 Hz
85 nV @ 16.7 Hz
Current: 400 μA typical
Power-down: 1 μA maximum
Low noise programmable gain instrumentation amp
Band gap reference with 4 ppm/°C drift typical
Update rate: 4.17 Hz to 470 Hz
3 differential inputs
Internal clock oscillator
Simultaneous 50 Hz/60 Hz rejection
Programmable current sources
On-chip bias voltage generator
Burnout currents
Power supply: 2.7 V to 5.25 V
–40°C to +105°C temperature range
Independent interface power supply
16-lead TSSOP package
Interface
www.DataSheet4U.com 3-wire serial
SPI®, QSPI™, MICROWIRE™, and DSP compatible
Schmitt trigger on SCLK
APPLICATIONS
Thermocouple measurements
RTD measurements
Thermistor measurements
Gas analysis
Industrial process control
Instrumentation
Portable instrumentation
Blood analysis
Smart transmitters
Liquid/gas chromatography
6-digit DVM
GENERAL DESCRIPTION
The AD7785 is a low power, low noise, complete analog front
end for high precision measurement applications. The AD7785
contains a low noise 20-bit ∑-Δ ADC with three differential
analog inputs. The on-chip, low noise instrumentation amplifier
means that signals of small amplitude can be interfaced directly
to the ADC. With a gain setting of 64, the rms noise is 40 nV
when the update rate equals 4.17 Hz.
The device contains a precision low noise, low drift internal
band gap reference and can accept an external differential
reference. Other on-chip features include programmable
excitation current sources, burnout currents, and a bias voltage
generator. The bias voltage generator sets the common-mode
voltage of a channel to AVDD/2.
The AD7785 can be operated with either the internal clock
or an external clock. The output data rate from the device is
software-programmable and can be varied from 4.17 Hz to 470
Hz. The device operates with a power supply from 2.7 V to
5.25 V. It consumes a current of 400 μA typical and is housed in
a 16-lead TSSOP package.
FUNCTIONAL BLOCK DIAGRAM
GND AVDD
REFIN(+)/AIN3(+) REFIN(–)/AIN3(–)
AIN1(+)
AIN1(–)
AIN2(+)
AIN2(–)
IOUT1
IOUT2
VBIAS
AVDD
BAND GAP
REFERENCE
MUX
BUF
IN-AMP
AVDD GND
INTERNAL
CLOCK
GND
Σ-Δ
ADC
SERIAL
INTERFACE
AND
CONTROL
LOGIC
AD7785
CLK
Figure 1.
DOUT/RDY
DIN
SCLK
CS
DVDD
Rev. 0
Information furnished by Analog Devices is believed to be accurate and reliable. However, no
responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other
rights of third parties that may result from its use. Specifications subject to change without notice. No
license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
Trademarksandregisteredtrademarksarethepropertyoftheirrespectiveowners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
www.analog.com
Fax: 781.461.3113
©2007 Analog Devices, Inc. All rights reserved.
1 page  
AD7785
Parameter
SCLK, CLK, and DIN (Schmitt-
Triggered Input)2
VT(+)
VT(–)
VT(+) − VT(−)
VT(+)
VT(–)
VT(+) − VT(−)
Input Currents
Input Capacitance
LOGIC OUTPUTS (INCLUDING CLK)
VOH, Output High Voltage2
VOL, Output Low Voltage2
VOH, Output High Voltage2
VOL, Output Low Voltage2
Floating-State Leakage Current
Floating-State Output Capacitance
Data Output Coding
SYSTEM CALIBRATION2
Full-Scale Calibration Limit
Zero-Scale Calibration Limit
Input Span
www.DataSheet4U.com POWER REQUIREMENTS7
Power Supply Voltage
AVDD to GND
DVDD to GND
Power Supply Currents
IDD Current
IDD (Power-Down Mode)
AD7785B 1
1.4/2
0.8/1.7
0.1/0.17
0.9/2
0.4/1.35
0.06/0.13
±10
10
DVDD − 0.6
0.4
4
0.4
±10
10
Offset binary
+1.05 × FS
−1.05 × FS
0.8 × FS
2.1 × FS
2.7/5.25
2.7/5.25
140
185
400
500
1
Unit Test Conditions/Comments
V min/V max
V min/V max
V min/V max
V min/V max
V min/V max
V min/V max
μA max
pF typ
V min
V max
V min
V max
μA max
pF typ
DVDD = 5 V
DVDD = 5 V
DVDD = 5 V
DVDD = 3 V
DVDD = 3 V
DVDD = 3 V
VIN = DVDD or GND
All digital inputs
DVDD = 3 V, ISOURCE = 100 μA
DVDD = 3 V, ISINK = 100 μA
DVDD = 5 V, ISOURCE = 200 μA
DVDD = 5 V, ISINK = 1.6 mA (DOUT/RDY)/
800 μA (CLK)
V max
V min
V min
V max
V min/max
V min/max
μA max
μA max
μA max
μA max
μA max
110 μA typ @ AVDD = 3 V, 125 μA typ @ AVDD = 5 V,
unbuffered mode, external reference
130 μA typ @ AVDD = 3 V, 165 μA typ @ AVDD = 5 V,
buffered mode, gain = 1 or 2, external reference
300 μA typ @ AVDD = 3 V, 350 μA typ @ AVDD = 5 V,
gain = 4 to 128, external reference
400 μA typ @ AVDD = 3 V, 450 μA typ @ AVDD = 5 V,
gain = 4 to 128, internal reference
1 Temperature range is –40°C to +105°C.
2 Specification is not production tested, but is supported by characterization data at initial product release.
3 Following a calibration, this error is in the order of the noise for the programmed gain and update rate selected.
4 Recalibration at any temperature removes these errors.
5 Full-scale error applies to both positive and negative full-scale and applies at the factory calibration conditions (AVDD = 4 V, gain = 1, TA = 25°C).
6 FS[3:0] are the four bits used in the mode register to select the output word rate.
7 Digital inputs equal to DVDD or GND with excitation currents and bias voltage generator disabled.
Rev. 0 | Page 5 of 32
5 Page 
AD7785
OUTPUT NOISE AND RESOLUTION SPECIFICATIONS
EXTERNAL REFERENCE
Table 5 shows the output rms noise of the AD7785 for some of
the update rates and gain settings. The numbers given are for
the bipolar input range with an external 2.5 V reference. These
numbers are typical and are generated with a differential input
voltage of 0 V. Table 6 shows the effective resolution, with the
output peak-to-peak (p-p) resolution shown in parentheses. It is
important to note that the effective resolution is calculated
using the rms noise, while the p-p resolution is based on the p-p
noise. The p-p resolution represents the resolution for which
there is no code flicker. These numbers are typical and are
rounded to the nearest LSB.
Table 5. Output RMS Noise (μV) vs. Gain and Output Update Rate Using an External 2.5 V Reference
Update Rate (Hz) Gain of 1 Gain of 2 Gain of 4 Gain of 8 Gain of 16 Gain of 32
4.17
0.64 0.6
0.29 0.22 0.1
0.065
8.33
1.04 0.96 0.38 0.26 0.13
0.078
16.7
1.55 1.45 0.54 0.36 0.18
0.11
33.2 2.3 2.13 0.74 0.5 0.23 0.17
62
2.95 2.85 0.92 0.58 0.29
0.2
123
4.89 4.74 1.49 1
0.48 0.32
242
11.76
9.5
4.02 1.96 0.88
0.45
470
11.33
9.44
3.07
1.79
0.99
0.63
Gain of 64
0.039
0.057
0.087
0.124
0.153
0.265
0.379
0.568
Gain of 128
0.041
0.055
0.086
0.118
0.144
0.283
0.397
0.593
Table 6. Typical Resolution (Bits) vs. Gain and Output Update Rate Using an External 2.5 V Reference
Update Rate (Hz) Gain of 1 Gain of 2 Gain of 4 Gain of 8 Gain of 16 Gain of 32 Gain of 64
4.17
www.DataSheet4U.com
8.33
20 (20)
20 (19.5)
20 (19.5)
20 (19)
20 (19.5)
20 (19)
20 (19)
20 (18.5)
20 (19)
20 (18.5)
20 (18.5)
20 (18.5)
20 (18.5)
20 (18)
16.7
20 (19)
20 (18)
20 (18.5) 20 (18)
20 (18)
20 (18)
20 (17.5)
33.2
20 (18.5) 20 (17.5) 20 (18)
20 (17.5) 20 (18)
20 (17.5)
19 (16.5)
62
20 (18)
19.5 (17) 20 (18)
20 (17.5) 20 (17.5)
19.5 (17)
19 (16.5)
123
20 (17.5) 19 (16.5) 19.5 (17) 19 (16.5) 19.5 (17)
19 (16.5)
18 (15.5)
242
18.5 (16) 18 (15.5) 18 (15.5) 18 (15.5) 18.5 (16)
18.5 (16)
17.5 (15)
470
18.5 (16) 18 (15.5) 18.5 (16) 18.5 (16) 18 (15.5)
18 (15.5)
17 (14.5)
Gain of 128
20 (17.5)
19.5 (17)
19 (16.5)
18.5 (16)
18 (15.5)
17 (14.5)
16.5 (14)
16 (13.5)
Rev. 0 | Page 11 of 32
11 Page | ||
| Páginas | Total 30 Páginas | |
| PDF Descargar | [ Datasheet AD7785.PDF ] | |
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