Dec 2002 Programmable Quad Supervisors Offer Unparalleled Flexibility for Multi-Voltage Monitoring Applications

DESIGN FEATURES
Programmable Quad Supervisors
Offer Unparalleled Flexibility for MultiVoltage Monitoring Applications
by Bob Jurgilewicz
Introduction
Three new power supply supervisors
improve system reliability by offering
more accurate reset thresholds than
other supervisors on the market. They
also save design time, production
costs and board space with easy-touse, flexible interfaces and a low
external parts count.
The LTC2900, L TC2901 and
LTC2902 quad supervisors can simultaneously monitor four supply
voltages with 1.5% threshold accuracy over temperature. Each part
offers 16 user-selectable four-voltage
combinations from the following: 5V,
3.3V, 3V, 2.5V, 1.8V, 1.5V, +ADJ and
–ADJ. A simple external resistor
divider performs single-pin programming, eliminating the need to qualify,
source and stock different part num-
5V
3.3V
DC/DC
CONVERTER
SYSTEM
LOGIC
2.5V
1.8V
1
C1
0.1µF
C2
0.1µF
R1
59k
1%
R2
40.2k
1%
bers for different combinations of
supply voltages. All three parts are
configured for 5% power supply tolerance and the LTC2902 can also be
LTC2900
LTC2901
LTC2902
Programmable Input
Threshold Combinations
16
16
16
Threshold Accuracy
1.5%
1.5%
1.5%
“Open-drain” Reset
LTC290x-1
●
●
●
Push-Pull Reset
LTC290x-2
●
●
●
Adjustable Reset Time
●
●
●
Buffered Reference
●
●
●
●
●
●
Independent Adjustable
Watchdog Circuitry
●
●
Reset Disable
Monitored Supply
Tolerance
Fixed
5%
Fixed
5%
User Selectable
5%, 7.5%, 10%,
12.5%
Package
10-lead MSOP
16-lead SSOP
16-lead SSOP
Linear Technology Magazine • December 2002
7
4
VREF
RST
VPG
PBR
GND
CRT
6
3
5
CRT
47nF
PUSH-BUTTON
RESET
2900 TA01
Figure 1. Typical application using the LTC2900-2 for 4-line voltage monitoring
Feature
Manual Reset
8
tRST = 216ms
Table 1. LTC2900, LTC2901 and LTC2902 Feature Summary
Individual Comparator
Outputs
9
V3
V4
V1
LTC2900-2
10
V2
2
programmed to work with power supplies at 7.5%, 10% and 12.5%
tolerance. These new devices require
no software, no calibration and no
trimming. In some applications, they
These new devices require
no software, no calibration
and no trimming
can be used with no external components, saving additional board space
and cost. Available features include
manual reset, watchdog functions,
selectable supply tolerance and supply margining functions. The reset
and watchdog times are also user
adjustable via external capacitors.
The LTC2900, LTC2901 and
LTC2902 supervisors offer micropower operation, small size, high
accuracy and multiple reset output
options. The extensive integrated
functionality makes these devices easy
to design into multi-voltage supervisory applications. Table 1 shows a
feature summary for these devices.
Figure 1 shows a fixed quad application with push-button reset using the
LTC2900-2.
19
DESIGN FEATURES
Safe Beginnings: Generating
the Power-On Reset (POR)
Reliable operation in many systems
requires knowledge of when certain
power supplies have exceeded mini-
mum thresholds and have remained
stable for a specified period of time.
One way to provide that knowledge is
with a reliable Power-On Reset (POR)
signal generated from a highly accurate voltage monitor.
Why is Threshold Accuracy Important?
A system voltage margin specification must take three factors into
account: power-supply tolerance, IC supply voltage tolerance and
supervisor reset threshold accuracy. If a system is to work reliably, none
of these can be left out of the design equation. The roles of the powersupply voltage tolerance and the IC supply voltage tolerance are fairly
straightforward, but the role of supervisor accuracy in reliable system
design is not as obvious. In the simplest terms, diminished accuracy
corresponds to a system that must operate reliably over a wider voltage
range, complicating the system design; whereas improved accuracy
decreases the voltage margin required for reliable system operation,
simplifying the system design.
Consider a 5V system with a ±5% power supply tolerance band (see
the figure in this sidebar). System ICs powered by this supply must
operate reliably within this band (and a little more, as explained below).
The bottom of the supply tolerance band, at 4.75V (5V–5%), is the exact
voltage at which a perfectly accurate supervisor would generate a reset.
Such a perfectly accurate supervisor does not exist—the actual reset
threshold may vary over a specified band (±1.5% for the LTC2900,
LTC2901 and LTC2902 supervisors).
With this variation of reset threshold in mind, the nominal reset
threshold of the supervisor is set below the minimum supply voltage;
just enough so that the reset threshold band and the power supply
tolerance bands do not overlap. If the two bands do overlap, the
supervisor could generate a false or nuisance reset when the power
supply is actually within its specified tolerance band (say, at 4.8V).
The LTC2900, LTC2901 and LTC2902 have ±1.5% reset threshold
accuracy, so 5% thresholds are typically set to 6.5% below the nominal
input voltage. For the 5V input, the typical threshold is 4.675V, or 75mV
below the ideal threshold of 4.750V. The actual threshold is guaranteed
to lie in the band between 4.750V and 4.600V over temperature. The
powered system must work reliably down to the low end of the threshold
band, or risk malfunction before a reset signal is properly issued. In our
5V example, using
NOMINAL
5V
the 1.5% accurate
SUPPLY VOLTAGE
SUPPLY TOLERANCE
supervisor, the sysMINIMUM
tem ICs must work
RELIABLE
IDEAL
SYSTEM
SUPERVISOR
down to 4.6V. The
VOLTAGE
THRESHOLD
same system using
4.75V
–5%
a ±2.5% accurate
±1.5%
supervisor must THRESHOLD
4.675V
–6.5%
±2.5%
operate down to
BAND
THRESHOLD
4.6V
–8%
4.5V, increasing the
BAND
REGION OF POTENTIAL MALFUNCTION
required system voltWITH 2.5% MONITOR
4.5V
–10%
age margin, and the
probability of system
Improved undervoltage monitor threshold accuracy
translates to improved system reliability
malfunction.
20
A typical device that requires a
reliable POR signal is a microprocessor. The LTC2900, LTC2901 and
LTC2902 can prevent a processor from
executing instructions until all supply voltages have reached safe
thresholds, regardless of the power
supply turn-on characteristics. Furthermore, if any supply voltage falls
back below a threshold with sufficient duration and magnitude, the
reset command is reissued. Once the
voltage has returned above the threshold and has remained there for a
specified amount of time, the reset
line is released.
In order to firmly establish the
correct reset logic state, power must
get to the reset drive circuitry early in
the power-up phase. The LTC2900,
LTC2901 and LTC2902 supervisors
are powered automatically from the
greater of the voltages on the V1 and
V2 inputs. With V1 or V2 at 1V or
greater, the reset output is specified
to be a logic low of 0.3V (max) while
sinking 100µA.
One Chip Covers All Supply
Voltages: Single Pin
Programming
The LTC2900, LTC2901 and LTC2902
ICs give designers the freedom to
specify one chip for all supervisory
applications, even though the nominal supply voltages may not be
finalized. The desired input voltage
combination is selected by placing a
simple resistive divider between the
reference pin (VREF) and ground (GND)
and connecting the tap point to the
programming pin (VPG), as shown in
Figure 2. The programming process
occurs during power-up and is transparent to the user. Table 2 specifies
the recommended 1% resistor values
for programming the available input
combinations. The last column in
LTC2900
8
VREF
7
VPG
6
GND
R1
1%
R2
1%
Figure 2. Programming the voltage
monitoring modes (see table 2 for R1
and R2 values)
Linear Technology Magazine • December 2002
DESIGN FEATURES
Mode
V1 (V)
V2 (V)
V3 (V)
V4 (V)
R1 (kΩ
Ω)
R2 (kΩ
Ω)
VPG/VREF
0
5.0
3.3
ADJ
ADJ
Open
Short
0.000
1
5.0
3.3
ADJ
–ADJ
93.1
9.53
0.094
2
3.3
2.5
ADJ
ADJ
86.6
16.2
0.156
3
3.3
2.5
ADJ
–ADJ
78.7
22.1
0.219
4
3.3
2.5
1.5
ADJ
71.5
28.0
0.281
5
5.0
3.3
2.5
ADJ
66.5
34.8
0.344
6
5.0
3.3
2.5
1.8
59.0
40.2
0.406
7
5.0
3.3
2.5
1.5
53.6
47.5
0.469
8
5.0
3.0
2.5
ADJ
47.5
53.6
0.531
9
5.0
3.0
ADJ
ADJ
40.2
59.0
0.594
10
3.3
2.5
1.8
1.5
34.8
66.5
0.656
11
3.3
2.5
1.8
ADJ
28.0
71.5
0.719
12
3.3
2.5
1.8
–ADJ
22.1
78.7
0.781
13
5.0
3.3
1.8
–ADJ
16.2
86.6
0.844
14
5.0
3.3
1.8
ADJ
9.53
93.1
0.906
15
5.0
3.0
1.8
ADJ
Short
Open
1.000
Table 2 specifies optimum VPG/VREF
ratios (±0.01) to be used when programming with a ratiometric DAC.
Monitor Any Positive or
Negative Voltage: Configuring
the Adjustable Inputs
Voltages not explicitly listed in Table␣ 2
can be monitored using the positive
adjustable (ADJ) and negative adjustable (–ADJ) inputs. The positive
adjustable threshold available on the
V3 or V4 input is set to 0.5V. For the
majority of positive adjustable applications, the tap point on an external
resistive divider (R3, R4) placed between the positive voltage being
sensed and ground is connected to
the high impedance input on V3 or
V4. Figure 3 demonstrates a generic
setup for the positive adjustable application.
The negative adjustable threshold
available on the V4 input is tied to
ground. In negative adjustable applications, the tap point on an external
resistive divider (R3, R4) placed between the negative voltage being
sensed and VREF, is connected to the
high impedance input on V4. The
voltage on the VREF pin (1.210V nominal) provides the necessary and
accurate level shift required to operate near ground. The VREF pin can
source and sink up to 1mA of current
over the full temperature range –40°C
VTRIP
R3
1%
LTC2900, LTC2901 OR LTC2902
8
+
V3 OR V4
R4
1%
–
+
–
0.5V
Figure 3. Setting the positive adjustable trip
point, VTRIP = 0.5V(1 + R3/R4)
Linear Technology Magazine • December 2002
R4
1%
R3
1%
VREF
9 V4
LTC2900, LTC2901
OR LTC2902
–
+
VTRIP
Figure 4. Setting the negative adjustable trip
point (VTRIP = –VREF(R3/R4)
to 85°C. Figure 4 shows a generic
setup for the negative adjustable application.
It is also possible to monitor voltages between ground and +0.5V using
the positive adjustable inputs. Similar to the offset technique in the
negative adjustable application, tie a
resistor from VREF to the V3 or V4
input, and an appropriate resistor to
the monitored voltage.
Quality System Design:
Consider Threshold Accuracy
and Noise Sensitivity
System reliability depends on power
supply reset thresholds that remain
accurate over temperature and power
supply variations (see sidebar). All
LTC2900, LTC2901 and LTC2902
supervisor inputs have the same relative threshold accuracy: ±1.5% of the
nominal input voltage over temperature (see Figure 5).
In any supervisory application,
supply noise riding on the monitored
DC voltage can cause spurious resets, particularly when the monitored
voltage is already near the reset
threshold.
One commonly used, but problematic, solution to this problem is the
addition of hysteresis to the input
comparator. The amount of hysteresis is usually specified as a
percentage of the trip threshold, and
typically needs to be added to the
advertised accuracy of the part in
order to determine the true accuracy
on the trip threshold. This technique
degrades accuracy, and therefore is
1.5
TYPICAL THRESHOLD ACCURACY (%)
Table 2. Voltage Threshold Programming
1.0
0.5
0
–0.5
–1.0
–1.5
–60 –40 –20 0
20 40 60
TEMPERATURE (°C)
80
100
Figure 5. Typical threshold accuracy
vs temperature (LTC2900, LTC2901
and LTC2902)
21
DESIGN FEATURES
220
TA = 25°C
400
350
300
TYPICAL TRANSIENT DURATION (µs)
TYPICAL TRANSIENT DURATION (µs)
450
RESET OCCURS
ABOVE CURVE
250
200
150
100
50
INPUTS: V1, V2
0
1
10
100
0.1
RESET COMPARATOR OVERDRIVE VOLTAGE (% OF VRTX)
CRT, between the CRT pin and ground.
The value of this capacitor is determined from:
TA = 25°C
200
180
RESET OCCURS
ABOVE CURVE
160
CRT = tRST • 217 • 10–9
140
with CRT in Farads and tRST in seconds. Maximum reset timeout is
limited by the largest available lowleakage capacitor. The accuracy of
the time-out period is affected by
capacitor leakage and capacitor tolerance. To maintain timing accuracy,
capacitor leakage must be well below
the 2µA nominal charging current.
120
100
80
60
40
20
INPUTS: V3, V4
0
0.1
1
10
100
RESET COMPARATOR OVERDRIVE VOLTAGE (% OF VRTX)
Figure 6. Typical transient duration versus overdrive required to trip comparator
not used on the LTC2900, LTC2901
and LTC2902 supervisors.
Instead, two forms of noise filtering are employed to minimize spurious
resets while maintaining system accuracy.
The first line of defense used to
minimize the effect of noise is a proprietary tailoring of the comparator
transient response. Transient events
receive a form of electronic integration in the comparator and must be of
sufficient magnitude and duration to
cause the comparator to switch. Figure 6 illustrates the typical transient
duration versus comparator overdrive
(as a percentage of the trip threshold
VRTx) required to trip the comparators.
The second filtering method, which
is under user control (see next section), is the adjustment of the reset
time-out period (tRST) or reset “delay
time”. A capacitor (CRT) attached between the CRT pin and ground sets
the reset time-out period. When any
supply drops below its threshold, the
Reset Output Options and
Individual Comparator
Outputs
reset line is brought low. The reset
time-out counter starts once all inputs are back above threshold. The
counter is cleared whenever any input drops back below threshold. A
noisy input with frequency components of sufficient magnitude above
f␣ =␣ 1/t RST effectively holds the reset
line low, preventing oscillatory behavior on the reset line.
Although all four supply monitor
comparators have built-in glitch filtering, bypass capacitors on V1 and
V2 are recommended because the
greater of V1 or V2 is also the VCC for
the chip (a 0.1µF ceramic capacitor is
satisfactory in most applications).
Filter capacitors on the V3 and V4
inputs are allowed and recommended
in extremely noisy situations.
The reset output line is available in
two styles, open-drain (LTC2900-1,
LTC2901-1 and LTC2902-1) and push
pull (LTC2900-2, LTC2901-2 and
LTC2902-2). The open-drain output
actually contains a weak pull-up current source to the V2 input, so an
external pull-up resistor is only required when the output needs to pull
to a higher voltage and/or when the
reset output needs a fast rise time.
The open-drain output allows for
wired-OR connections and can be
useful when more than one signal
needs to pull down on the reset line.
The non-delayed individual comparator outputs available on the LTC2901
and LTC2902 also have open-drain
outputs with identical pull-up characteristics. When externally pulling
up to voltages higher than V2, an
internal network is automatically
enabled to protect the weak pull-up
circuitry from reverse currents.
User Adjustable Reset TimeOut Period
The reset time-out period (tRST) is
adjustable in order to accommodate a
variety of applications. The period is
adjusted by connecting a capacitor,
R5A
86.6k 1%
MASTER
RESET
R3B
464k 1%
–5V
2.5V
1V
R3A
2150k 1%
3V
1
12V
5V
2
V3
V2
10
V4
V1
LTC2900-2
8
VREF
CRT
4
7
RST
VPG
5
6
PBR
GND
1
1.8V
9
R6A
100k
1%
CRTA
20k
R4B
121k
1%
10
9
V4
V1
LTC2900-2
8
VREF
CRT
4
7
RST
VPG
6
5
PBR
GND
3.3V
3
3
R4A
100k
1%
V2
V3
2
R1A
40.2k
1%
R2A
59k
1%
CRTB
R1B
22.1k
1%
R2B
78.7k
1%
100k
2900 TA06
Figure 7. Two supervisors cascaded to monitor eight voltages
22
Linear Technology Magazine • December 2002
DESIGN FEATURES
The push-pull reset output has a
much stronger active pull-up capability, also to the V2 input, resulting
in a faster, low voltage drop pull-up
characteristic. Wired-OR connections
and/or external pull-ups are not recommended with the push-pull reset
output option.
Ensuring Reset Valid for VCC
down to 0V (LTC2900-2,
LTC2901-2 and LTC2902-2)
Some applications require the reset
output (RST) to be valid with VCC
down to 0V. The L TC2900-2,
LTC2901-2 and LTC2902-2 are designed to handle this requirement
with the addition of an external resistor from RST to ground. The resistor
will provide a path for stray charge
and/or leakage currents, preventing
the RST output from floating to undetermined voltages when connected to
high impedance (such as CMOS logic
inputs). The resistor value should be
small enough to provide effective pulldown without excessively loading the
active pull-up circuitry. Too large a
value may not pull-down well enough.
A 100k resistor from RST to ground is
satisfactory for most applications.
Manual Reset Feature on the
LTC2900
The manual reset or push-button reset pin (PBR) on the LTC2900 is used
to issue a forced reset, typically with
a normally-open pushbutton switch
attached between PBR and ground.
The PBR pin is pulled to VCC with an
internal current source of 10µA (typi-
cal). The switch is debounced through
the reset circuitry using the delay
provided by the CRT timing capacitor.
A logic low on this pin will pull RST
low. When the PBR pin returns high,
RST will return high after the reset
time-out period has elapsed, assuming all four voltage inputs are above
their thresholds. The PBR pin may
also be driven by a logic signal. The
input-high threshold on the PBR pin
is 1.6V (max), allowing the pin to be
driven by low-voltage logic. Figure 7
demonstrates a supervisory cascade
using two LTC2900-2 ICs to monitor
8 voltages. The reset output of the
first supervisor is tied to the PBR
input of the second which holds the
master reset low while the voltages on
the first supervisor are below threshold. When all eight voltages are above
threshold, the master reset is released
after the delay provided by the second
reset timing capacitor (CRTB).
Independent Watchdog
Features on the LTC2901
The LTC2901 contains independent
watchdog circuitry consisting of a
watchdog input (WDI), a watchdog
output (WDO) and a timing pin (CWT)
that allows for a user adjustable
watchdog time-out period. An undervoltage condition on any supervisor
input causes RST to go low which
clears the watchdog timer and brings
WDO high. The watchdog timer is
started when RST pulls high. Subsequent rising or falling edges received
on the WDI pin will clear the watchdog timer. If an edge is not received
LTC1772
6
ITH
PGATE
2
5
VIN
GND
3
4
VFB SENSE –
1
R6
10k
C3
220pF
C1: TAIYO YUDEN CERAMIC LMK325BJ106K-T
C2: SANYO POSCAP 6TPA47M
D1: MOTOROLA MBRM120T3
L1: COILTRONICS UP1B-100
M1: Si3443DV
R5: DALE 0.25W
M1
VIN
3.3V
C1
10µF
10V
R5
0.15Ω
CWT = tWD • 50 • 10–9
with CWT in Farads and tWD in seconds. Maximum timeout is limited by
the largest available low-leakage capacitor. The accuracy of the time-out
period is affected by capacitor leakage and capacitor tolerance. To
maintain timing accuracy, capacitor
leakage must be well below the 2µA
nominal charging current.
The watchdog circuit can also be
used as a clock or frequency monitor
by applying a periodic logic signal to
the WDI input. If the input signal is
4
14
3
L1
10µH
D1
within the watchdog time-out period,
WDO will go low. WDO will remain
low and the watchdog timer will remain cleared until another edge is
received on the WDI pin or another
undervoltage condition occurs.
The watchdog function is typically
used to monitor a processor’s activity. Consider a system that is no
longer executing the correct code,
thereby failing to issue an edge to the
WDI pin. If the watchdog output is
tied to a non-maskable interrupt
(NMI), a watchdog timeout will cause
the processor to vector to a new program location, which may enable a
variety of recovery actions. For example, a motor could be disabled, an
interlock could be engaged, critical
data could be written to NVRAM, etc.
The watchdog time-out period (tWD)
can be optimized for software execution. A capacitor (CWT) connected
between the CWT pin and ground sets
the watchdog time-out period. The
capacitor value is determined from:
+
C2
47µF
6V
R3
100k
R4
80.6k
13
VOUT
1.8V
0.5A
8
12
R1
28k
1%
R2
71.5k
1%
11
10
LTC2901-2
2
V1
COMP1
16
V2
COMP2
1
V3
COMP3
15
V4
COMP4
6
WDI
RST
7
VREF
WDO
5
CRT
VPG
9
CWT
GND
3.3V MONITOR
1.8V MONITOR
FEEDBACK MONITOR
COMMON RESET OUT
LOW LOAD INDICATOR
CRT
47nF
2901 F07
Figure 8. Use the LTC2901-2 to monitor the input, output, feedback voltage and low load conditions on a DC/DC
controller. In this case, the controller is an LT1772 used in a 3.3V input to 1.8V output application.
Linear Technology Magazine • December 2002
23
DESIGN FEATURES
inactive for an amount of time longer
than the watchdog time-out period,
the WDO line falls, indicating a loss of
the periodic input. Figure 8 demonstrates how the LTC2901 can be used
to monitor a switching regulator’s
activity. In this application, the 3.3V
input, 1.8V output and feedback voltage to the LTC1772 regulator are
supervised. Furthermore, if the load
goes open circuit, the LTC1772
switches into Burst Mode® operation,
reducing the duty cycle at the gate of
M1. The pulse spacing exceeds the
watchdog time-out period, and the
watchdog output falls indicating the
low-load condition.
Power Supply Margin Testing
with the LTC2902
In high reliability system manufacturing and test, it is desirable to verify
the correct operation of electrical components at or below the rated power
supply tolerance. The LTC2902 is
designed to complement such testing
in two ways. First, the reset disable
pin (RDIS) can be pulled low which
forces the RST output high. With RDIS
low, moving supply voltages below
threshold does not invoke the reset
command during margining tests. The
individual comparator outputs operate normally with RDIS high or low,
allowing for individual supply monitoring.
LTC6910-1, continued from page 18
frequency corner of 1Hz, which can
be adjusted by changing C1. Alternatively, shorting C1 makes the amplifier
DC-coupled. (When DC is not needed,
however, the AC coupling suppresses
low frequency noise and all amplifier
offset voltages other than the low
internally-trimmed LT1884 offset in
the integrating amplifier, which is the
second amplifier in Figure 6. If desired, another coupling capacitor in
series with the input can relax the
requirements on input DC level as
well.)
24
10k
Table 3. LTC2902 Tolerance Programming
6
5V
3.3V
2.5V
1.8V
R1
59k
1%
R2
40.2k
1%
4
14
V1
RST
V2
T0
T1
7
T0
T1
Tolerance
VREF
9
Low
Low
5%
1.210V
Low
High
7.5%
1.178V
High
Low
10%
1.146V
High
High
12.5%
1.113V
3
8
V3
RDIS
LTC2902-1
13
2
V4
COMP1
16
12
COMP2
VREF
1
COMP3
11
15
VPG
COMP4
GND
10
CRT
5
CRT
47nF
Figure 9. Quad supply monitor
with asymmetric hysteresis
metric hysteresis, having 5% tolerance when supplies are rising and
12.5% tolerance after all supplies have
safely crossed their 5% thresholds.
Conclusion
The second way allows the user to
provide more supply headroom by
lowering the trip thresholds. Using
the digital tolerance programming
inputs (T0, T1), the global supply
tolerance can be set to 5%, 7.5%,
10%, or 12.5% (Table 3).
When using the positive or negative adjustable inputs in conjunction
with tolerance programming, external resistors need only to be sized
once, based on a 5% tolerance threshold. Once the external resistor dividers
are set using the 5% tolerance thresholds, the thresholds for the other
tolerance modes (7.5%, 10%, 12.5%)
are automatically correct because the
reference voltage (VREF) is scaled accordingly. Figure 9 shows how the
LTC2902 can be configured for asym-
One part can now satisfy most present
and future supervisory needs. The
LTC2900, LTC2901 and LTC2902
micropower quad supervisors provide
the versatility, accuracy and reliability
required in multi-voltage monitoring
applications. Input supply combinations are programmable including
positive and/or negative adjustable
thresholds. The comparators are 1.5%
accurate over temperature and feature built-in noise rejection. Reset
logic is correct for VCC down to 1V,
and is available with open-drain or
push-pull outputs. Reset and watchdog times are user adjustable with
external capacitors. Power supply
margining features include real-time
supply tolerance selection and an ondemand reset disable pin.
Measured frequency responses
(Figure 8) demonstrate bandwidth
settings of 10Hz, 100Hz, and 1kHz
(digital BW inputs of 001, 100, and
111, respectively) and unity gain in
each case. By scaling C2, this circuit
can serve other frequency ranges,
such as a maximum of 10kHz with
0.1µF using LT1884 (gain-bandwidth
product around 1MHz). Output signal-to-noise ratio measured with
10mVP–P input, gain of 100, and 100Hz
bandwidth is 76dB; for 100mVP–P
input, gain of 10, and 1000Hz bandwidth it is 64dB.
Conclusion
With a printed circuit footprint of only
about 11mm 2 , the easy-to-use
LTC6910-1 provides two decades of
programmable DC or AC voltage gain.
It can preamplify, drive loads, and
introduce gain flexibility into spaces
so small that, as one engineer put it,
“your boss doesn’t even need to know
it’s there.”
Acknowledgements
Mark Thoren and Derek Redmayne collaborated
on the ADC application and Philip Karantzalis
contributed the AC amplifier.
Linear Technology Magazine • December 2002
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