UG-513: Evaluating the AD8237 Micropower, Zero Drift, True Rail-to-Rail...

EVAL-INAMP-ICF-RMZ User Guide
UG-513
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com
Evaluating the AD8237 Micropower, Zero Drift, True Rail-to-Rail Instrumentation
Amplifier and the AD8420 Wide Supply Range, Micropower, Rail-to-Rail
Instrumentation Amplifier
FEATURES
jumpers are included for added convenience. The board also offers
an SMA/SMB outline at the input and output for best signal integrity.
Simple evaluation of AD8237 and AD8420
SMA/SMB or vector test pin inputs and outputs
Bandwidth mode selection switch for AD8237
Shipped with AD8420ARMZ and AD8237ARMZ
Users can add their own end-launch SMA/SMB connector to
interface the evaluation board with complementary tools, such as
an analog-to-digital converter (ADC) evaluation board, available
from Analog Devices, Inc.
To get started with this evaluation board, see the Quick Start for
the EVAL-INAMP-ICF-RMZ Board section.
APPLICATIONS
Quick, easy product evaluation and characterization
Pads for common surrounding components
Interface to other evaluation tools
BASIC OPERATION
GENERAL DESCRIPTION
The EVAL-INAMP-ICF-RMZ allows users the option of evaluating
the AD8237 or the AD8420 indirect current feedback (ICF)
instrumentation amplifier individually or connected to an adjacent
board in an application. This evaluation board ships with both
instrumentation amplifiers in surface-mount MSOP packages.
The evaluation board is arranged so that the user can easily adjust
the gain resistors and common discrete components surrounding
the part. A bandwidth mode switch for the AD8237 and reference
The AD8237 and AD8420 series in-amps convert a differential
input signal to a gained single-ended output signal with respect
to a reference voltage. The board connects the user-provided
power supplies to pins of the integrated circuit and ground to
the board. The differential signal is applied between the +IN
and −IN pins of the in-amp, while the signal output appears
between the VOUT pin and the REF pin of the in-amp.
Figure 2 shows the component layout and the location of the
connections for the EVAL-INAMP-ICF-RMZ printed circuit
board (PCB).
11214-001
DIGITAL PHOTOGRAPH OF THE EVAL-INAMP-ICF-RMZ
Figure 1. EVAL-INAMP-ICF-RMZ Evaluation Board, Primary Side
PLEASE SEE THE LAST PAGE FOR AN IMPORTANT
WARNING AND LEGAL TERMS AND CONDITIONS.
Rev. 0 | Page 1 of 8
UG-513
EVAL-INAMP-ICF-RMZ User Guide
TABLE OF CONTENTS
Features .............................................................................................. 1
Quick Start for the EVAL-INAMP-ICF-RMZ Board ...................4
Applications ....................................................................................... 1
Circuit Options ..............................................................................4
General Description ......................................................................... 1
Reference ........................................................................................6
Basic Operation................................................................................. 1
Input ................................................................................................6
Digital Photograph of the EVAL-INAMP-ICF-RMZ .................. 1
Output .............................................................................................7
Revision History ............................................................................... 2
EVAL-INAMP-ICF-RMZ Schematic and User Options ............. 3
User Options ................................................................................. 3
REVISION HISTORY
2/13—Revision 0: Initial Version
Rev. 0 | Page 2 of 8
EVAL-INAMP-ICF-RMZ User Guide
UG-513
EVAL-INAMP-ICF-RMZ SCHEMATIC AND USER OPTIONS
VCC
C4
0.1µF
VEE
C5
0.1µF
S1
5
C1
R1
+IN
1
2
U1
C3
R2
–IN
3
VINN
OUT
4
+
VINP
C2
FB
–
REF
VOUTP
8
7
RF
6
CF1
FB1
R3
VEE
RG
GND1
+
C7
10µF
+
GND2
GND
R5
FB2
REF
R4
W1
C6
10µF
W2
VOUTN
11214-002
VCC
Figure 2. EVAL-INAMP-ICF-RMZ Schematic Diagram
USER OPTIONS
Table 1. Jumpers and Switches
Jumper
W1
Function
REF to GND
W2
VOUTN select
S1
Pin 1 select
Description
REF is shorted to ground through W1 solder jumper. Remove W1 to drive REF with a voltage
other than ground.
Determines what voltage is at VOUTN. If REF is being driven to level-shift the output but
GND must be passed to an adjacent board through the SMA/SMB, remove the solder jumper
and set W2 to GND. Otherwise, it can be left as-is.
Bandwidth mode setting for the AD8237. If the switch toggle is moved to the right (LO), Pin 1 is
shorted to VEE (AD8237 low bandwidth mode). If the switch toggle is moved to the left position
(HI), Pin 1 is shorted to VCC (AD8237 high bandwidth mode). For the AD8420, the best frequency
response is obtained if S1 is set to LO (Pin 1 is shorted to VEE).
Factory Setting
Shorted
REF
LO (−VS)
Table 2. Input/Output
I/O
VCC
VEE
GND
+IN/VINP
−IN/VINN
OUT/VOUTP
FB1/FB2
REF
VOUTN
Function
Positive supply voltage rail of the in-amp.
Negative supply voltage rail of the in-amp.
Ground connection for the board. Connect to power supply ground. GND1, GND2, and GND pins are all connected to board
ground.
Positive input of the in-amp (vector pin/SMA).
Negative input of the in-amp (vector pin/SMA).
Output of the in-amp (vector pin/SMA).
Feedback input for the in-amp. Two pins are provided to simplify gain setting with leaded resistors on press mount sockets.
External reference input for the in-amp. W1 must be removed before driving REF with a voltage source.
Selectable reference or ground SMA output (see W2 in Table 1).
Table 3. Suggested Accessories
Reference Designation
SMA Jack
SMA Plug
SMB Jack
SMB Plug
Press mount socket
Part
Side launch 0.062” board thickness, such as the EMERSON 142-0701-851 (Digi-Key J658-ND)
Side launch 0.062” board thickness, such as the EMERSON 142-0801-811 (Digi-Key J10131-ND)
Side launch 0.062” board thickness, such as the EMERSON 131-3701-801 (Digi-Key J10107-ND)
Recommend above jack and adapter, such as the Amphenol Connex 142246 (Digi-Key ACX1479-ND)
Plugs onto the vector test pin, such as the Cambion 450-4352-01-03-10 (Newark 40F6130). Can be used for
inputs and outputs or through-hole gain setting resistors.
Rev. 0 | Page 3 of 8
UG-513
EVAL-INAMP-ICF-RMZ User Guide
QUICK START FOR THE EVAL-INAMP-ICF-RMZ BOARD
VCC
10µF
3.
4.
Single-Supply Range
1.8 V to 5.5 V
2.7 V to 36 V
Dual-Supply Range
±0.9 V to ±2.75 V
±2.7 V1 to ±18 V
For VCM, VREF = 0 V. The ±1.35 V supply may be used with VCM and VREF at VEE.
Table 6. Input Range
Part Number
AD8237
AD8420
1
2
R2 3
Input Range
(+IN, –IN, FB, and REF)
VEE − 0.3 V to VCC + 0.3 V
VEE − 0.15 V to VCC − 2.2 V
5
+
1
4
DUT
–
10µF
0.1µF
OUT
8
FB
REF
7
RF
VOUTP
0Ω
+
VOUT
6
RG OPEN REF –
W1
W2
VOUTN
Figure 3. Default Connection Diagram
Table 5. Supply Voltage Range
1
0Ω
Purpose
REF tied to ground
Ensures proper setting for AD8237
Install the desired component in the MSOP footprint.
Connect the supply voltages within the ranges shown in
Table 5 to VCC and VEE, and connect the power supply
ground return to GND.
Drive the inputs with a signal within the listed input range
shown in Table 6. Ensure that the expected output signal
(default configuration: VOUT = V+IN − V−IN) is within the
supplies.
Measure the output on a multimeter or oscilloscope. The
output voltage is the voltage measured between VOUT (OUT)
and VREF (REF). The default configuration is a gain of 1
with REF shorted to the board ground. The output can
drive another device, such as another amplifier or an ADC.
Part Number
AD8237
AD8420
R1 2
0Ω
–IN
Follow Step 1 through Step 4 to get started using the EVALINAMP-ICF-RMZ evaluation board (see Figure 3).
1.
2.
+IN
VINN
Table 4. Factory Setting
Connection
W1 Soldered
S1 set to LO
S1
0.1µF
SMA/SMB
CONNECTORS
NOT INSTALLED
VINP
VEE
11214-003
The quick start instructions in this section assume that the
evaluation board is in its default condition as is shown in Table 4.
The values provided are intended to simplify the process of getting
started and of checking functionality. Consult the AD8237 data
sheet and the AD8420 data sheet for more detailed and up-to-date
information on these instrumentation amplifiers.
Differential
Input Limit1
±(VS − 1.2)2
±1 V
Defined as the maximum voltage between +IN and −IN for the valid output.
For example, if VCC = 2.5 V and VEE = −2.5 V, the differential input limit is ±3.8 V.
CIRCUIT OPTIONS
Using a Single Supply
The EVAL-INAMP-ICF-RMZ can be used in single-supply mode
by shorting the VEE pin to ground and connecting the single
power supply between VCC and ground. Both the AD8237 and
the AD8420 can operate with the inputs and the reference pin at
ground on a single supply; however, because the output cannot
swing all the way to ground, there must be a positive input signal.
For this reason, in many single-supply circuits, it is necessary to
remove W1 and drive the reference pin with a higher voltage, such
as midsupply. See the Reference section for more details.
Changing the Gain
The EVAL-INAMP-ICF-RMZ board defaults to a gain of 1 because
of the 0 Ω resistor at RF. Removing the 0 Ω resistor and installing
0805-sized resistors at RF and RG creates the following transfer
function:
VOUT = G(V+IN − V−IN) + VREF
where G = 1 + (RF/RG).
Table 7. Suggested 1% Resistor Values for Various Gains
RF (kΩ)
Short
49.9
80.6
90.9
95.3
97.6
100
200
499
1000
RG (kΩ)
None
49.9
20
10
5
2
1
1
1
1
Gain
1.00
2.00
5.03
10.09
20.06
49.8
101
201
500
1001
While the ratio of RF to RG sets the gain, the designer determines
the absolute value of the resistors. Larger values reduce power
consumption and output loading; smaller values limit the FB input
bias current or offset current error. For best performance, keep
(RF + RG) || RL ≥ 20 kΩ.
Rev. 0 | Page 4 of 8
EVAL-INAMP-ICF-RMZ User Guide
UG-513
Compensating Errors from the FB Pin Bias Current and
Input Impedance
RFI Filtering
Cutting the trace between the R4 pads and placing a resistor of
RF || RG in R4 limits errors from the FB pin bias current (see
Figure 4). At higher gains, this resistor can be the same value as RG.
Although the offset current of the AD8237 is the same as the bias
current, this resistor still helps to compensate the error due to the
common-mode input resistance and is therefore recommended.
An RFI filter is formed by R1 and R2 with C1, C2, and C3. RFI
signals can be filtered with a low-pass RC network placed at the
input of the instrumentation amplifier, such as the one shown in
Figure 6, where C1 = C2 = CC, C3 = CD, and R1 = R2 = R. The
filter limits the input signal bandwidth according to the following
relationship:
FilterFrequency DIFF =
+IN
VOUT
IB+
FilterFrequency CM =
AD8420
FB
IBF
REF
–IN
RF
where CD ≥ 10 CC.
IBR
RG
R4
= RF||RG
RF
RG
VCC
VREF
R
20kΩ
1%
Figure 4. Cancelling Out Error from FB Input Bias Current
RS
R
CD
20kΩ 3300pF
1%
RIN
VIN
FB
R3
RF
IF RF||RG + R3 = RS,
VOUT = VIN × (1 +
RF
)
RG
11214-005
RG
–IN
CC
330pF
5%
RG
RF
10µF
0.1µF
VEE
CD affects the differential signal, and CC affects the common-mode
signal. Values of R and CC are chosen to minimize out of band RFI
at the expense of reduced signal bandwidth. Mismatch between
the R × CC at the positive input and the R × CC at the negative input
degrades the CMRR of the in-amp. By using a value of CD that is
at least one magnitude larger than CC, the effect of the mismatch is
reduced and performance is improved.
VOUT
RIN
REF
–IN
VOUT
AD8420
Figure 6. RFI Filtering
RIN
RS + RIN
AD8237
+IN
CC
330pF
5%
+IN
If the source resistance is well known, setting the parallel
combination of RF and RG equal to RS can be used to cancel
differential input impedance errors. If practical resistor values
force the parallel combination of RF and RG to be less than RS, cut
the trace between the R3 pads and insert a resistor to make up
for the difference.
V+IN = VIN ×
10µF
0.1µF
11214-004
G=1+
1
2πRC C
11214-006
IB–
1
2πR(2C D + C C )
The AD8237 has an internal RFI filter that is sufficient for most
applications. Nonetheless, an external RFI filter can be added if
additional immunity is necessary.
Figure 5. Canceling Input Impedance Errors
Mode Switch
By default, S1 is switched to the right (LO). For AD8237, this
corresponds to low bandwidth mode. In low bandwidth mode,
AD8237 is unity-gain stable and has a gain bandwidth product
of 200 kHz. For gains greater than or equal to 10, S1 can be
switched to the left (HI) to put the AD8237 in high bandwidth
mode, which has a gain bandwidth product of 1 MHz. In high
bandwidth mode, AD8237 is not stable for gains less than 10.
R1 and R2 can also be used for input protection or to simulate
the source resistance expected in an application. Alternatively,
input attenuators can be formed with R1 and a resistor in the place
of C1, and/or R2 and a resistor in the place of C2. A differential
attenuator can be formed with R1, a resistor in the C3 position,
and R2.
For the AD8420, Pin 1 is listed as a no connect pin; however, it
is high impedance and may be shorted to either supply. Setting
S1 to LO matches the parasitic capacitance at the inputs, allowing
the best common-mode rejection ratio (CMRR) vs. frequency.
Rev. 0 | Page 5 of 8
UG-513
EVAL-INAMP-ICF-RMZ User Guide
Other Board Options
INPUT
A capacitor in CF1, which is parallel with RF, can be used to
reduce the gain at high frequencies and therefore reduce noise.
Do not leave the in-amp inputs floating, that is, without a dc return
path for the bias current. This often occurs when the inputs are
connected to a transformer, a thermocouple, or a pair of series
capacitors. The inputs must have a dc path to ground, as shown
in Figure 7 and Figure 8.
If R5 is installed instead of RG, the following transfer function
results:
VOUT = G(V+IN − V−IN + VREF)
The circuit shown in Figure 8 has series capacitors, C, between
the signal generator and the input. The series capacitors act as an
open circuit at dc; therefore, they prevent a dc current from flowing
into the input transistors of the in-amp. Two 10 kΩ resistors are
used between the inputs of the in-amp and ground to provide the
necessary current path. Closely match the resistors and capacitors
in the positive and negative signal path to optimize CMRR.
Where G = 1 + (RF/R5)
In this case, VOUT is measured with respect to ground. This transfer
function can be useful in certain cases, such as electrocardio gram
(ECG) circuits that require high gain and dc removal. This
configuration on the evaluation board is generally most useful
when a dual supply is used. See the Applications Information
section for the AD8420 data sheet or the AD8237 data sheet for
more information.
REFERENCE
The reference level of the instrumentation amplifier can be set
by driving the REF pin with an amplifier or voltage reference, a
dc source, or even a resistor divider (see the AD8420 data sheet
or the AD8237 data sheet for further information). By default,
W1 is in place and REF is shorted to ground. If a reference voltage
other than ground is used, remove the solder from W1 before
driving REF with the reference voltage, as shown in Figure 7.
VCC
10µF
S1
0.1µF
5
2
+IN
+
3
0.1µF
10µF
4
OUT
8
FB
REF
6
–
+COM
1
DUT
100mV p-p, 1kHz
–IN
OSCILLOSCOPE
7
RF
RG
97.6kΩ
2.49kΩ
REF
W1
11214-007
5V
VEE
2.5V
Figure 7. Using an External Voltage Source to Set the Reference Level (See Encircled)
VCC
VEE
OSCILLOSCOPE
S1
0.1µF
5
C
2
+IN
+
–IN
3
10kΩ
–
4
REF
6
10kΩ
10µF
+COM
1
DUT
C
0.1µF
OUT
8
FB
7
RF
RG
REF
11214-008
10µF
W1
Figure 8. AC Input Coupling, Using a Resistive DC Return Path
Rev. 0 | Page 6 of 8
EVAL-INAMP-ICF-RMZ User Guide
UG-513
VCC
Measure the output voltage by monitoring the voltage between the
OUT and REF tie points of the board, as shown in Figure 9. If
an external voltage reference is used, the output can be measured,
referred to ground, as shown in Figure 7. The evaluation board
offers two SMA/SMB outlines at the output, VOUTP and VOUTN.
The cable shields for VOUTP and VOUTN are both connected
to ground. The voltage in the signal conductor of VOUTN is
selectable by Solder Jumper W2. By default, the output voltage at
VOUTN is VREF. If the REF pin is being driven but ground must be
passed to the next stage, such as a pseudo-differential ADC, the
default solder jumper can be removed, and VOUTN can be shorted
to ground. Take care that the voltage source driving REF does not
become inadvertently shorted to ground during this process.
10µF
2
+
Rev. 0 | Page 7 of 8
3
–
0.1µF
10µF
+COM
1
4
DUT
–IN
OSCILLOSCOPE
S1
0.1µF
5
+IN
VEE
OUT
8
FB
REF
6
7
RF
RG
REF
W1
Figure 9. Correct Output Connection Using the REF Pin
11214-009
OUTPUT
UG-513
EVAL-INAMP-ICF-RMZ User Guide
NOTES
ESD Caution
ESD (electrostatic discharge) sensitive device. Charged devices and circuit boards can discharge without detection. Although this product features patented or proprietary protection
circuitry, damage may occur on devices subjected to high energy ESD. Therefore, proper ESD precautions should be taken to avoid performance degradation or loss of functionality.
Legal Terms and Conditions
By using the evaluation board discussed herein (together with any tools, components documentation or support materials, the “Evaluation Board”), you are agreeing to be bound by the terms and conditions
set forth below (“Agreement”) unless you have purchased the Evaluation Board, in which case the Analog Devices Standard Terms and Conditions of Sale shall govern. Do not use the Evaluation Board until you
have read and agreed to the Agreement. Your use of the Evaluation Board shall signify your acceptance of the Agreement. This Agreement is made by and between you (“Customer”) and Analog Devices, Inc.
(“ADI”), with its principal place of business at One Technology Way, Norwood, MA 02062, USA. Subject to the terms and conditions of the Agreement, ADI hereby grants to Customer a free, limited, personal,
temporary, non-exclusive, non-sublicensable, non-transferable license to use the Evaluation Board FOR EVALUATION PURPOSES ONLY. Customer understands and agrees that the Evaluation Board is provided
for the sole and exclusive purpose referenced above, and agrees not to use the Evaluation Board for any other purpose. Furthermore, the license granted is expressly made subject to the following additional
limitations: Customer shall not (i) rent, lease, display, sell, transfer, assign, sublicense, or distribute the Evaluation Board; and (ii) permit any Third Party to access the Evaluation Board. As used herein, the term
“Third Party” includes any entity other than ADI, Customer, their employees, affiliates and in-house consultants. The Evaluation Board is NOT sold to Customer; all rights not expressly granted herein, including
ownership of the Evaluation Board, are reserved by ADI. CONFIDENTIALITY. This Agreement and the Evaluation Board shall all be considered the confidential and proprietary information of ADI. Customer may
not disclose or transfer any portion of the Evaluation Board to any other party for any reason. Upon discontinuation of use of the Evaluation Board or termination of this Agreement, Customer agrees to
promptly return the Evaluation Board to ADI. ADDITIONAL RESTRICTIONS. Customer may not disassemble, decompile or reverse engineer chips on the Evaluation Board. Customer shall inform ADI of any
occurred damages or any modifications or alterations it makes to the Evaluation Board, including but not limited to soldering or any other activity that affects the material content of the Evaluation Board.
Modifications to the Evaluation Board must comply with applicable law, including but not limited to the RoHS Directive. TERMINATION. ADI may terminate this Agreement at any time upon giving written notice
to Customer. Customer agrees to return to ADI the Evaluation Board at that time. LIMITATION OF LIABILITY. THE EVALUATION BOARD PROVIDED HEREUNDER IS PROVIDED “AS IS” AND ADI MAKES NO
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©2013 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
UG11214-0-2/13(0)
Rev. 0 | Page 8 of 8