Datasheet IVC102U, IVC102P, IVC102U-2K5 Datasheet (Burr Brown)

1

®

IVC102

I

IN

V

B

1

2

3

4

5

6

11 12 13

Digital

Ground

Analog
Ground

Logic Low closes switches

9

10

14

V

O

V+

V–

S

1

S

2

Ionization

Chamber

Photodiode

60pF

30pF

10pF

S

1

C

1

C

2

C

3

S

2

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Tel: [phone] • Twx: 910-952-1111 • Cable: BBRCORP • Telex: 066-6491 • FAX: [phone] • Immediate Product Info: [phone]

®

PRECISION SWITCHED INTEGRATOR

TRANSIMPEDANCE AMPLIFIER

APPLICA TIONS

● PRECISION LOW CURRENT MEASUREMENT

● PHOTODIODE MEASUREMENTS

● IONIZATION CHAMBER MEASUREMENTS

● CURRENT/CHARGE-OUTPUT SENSORS

● LEAKAGE CURRENT MEASUREMENT

IVC102

FEATURES

● ON-CHIP INTEGRATING CAPACITORS

● GAIN PROGRAMMED BY TIMING

● LOW INPUT BIAS CURRENT: 750fA max

● LOW NOISE

● LOW SWITCH CHARGE INJECTION

● FAST PULSE INTEGRATION

● LOW NONLINEARITY: 0.005% typ

● 14-PIN DIP, SO-14 SURFACE MOUNT

DESCRIPTION

The IVC102 is a precision integrating amplifier with
FET op amp, integrating capacitors, and low leakage
FET switches. It integrates low-level input current for
a user-determined period, storing the resulting voltage
on the integrating capacitor. The output voltage can be
held for accurate measurement. The IVC102 provides
a precision, lower noise alternative to conventional
transimpedance op amp circuits that require a very
high value feedback resistor.

The IVC102 is ideal for amplifying low-level sensor
currents from photodiodes and ionization chambers.
The input signal current can be positive or negative.

TTL/CMOS-compatible timing inputs control the inte­gration period, hold and reset functions to set the
effective transimpedance gain and to reset (discharge)
the integrator capacitor.

Package options include 14-Pin plastic DIP and SO-14
surface-mount packages. Both are specified for the
–40°C to 85°C industrial temperature range.

© 1996 Burr-Brown Corporation PDS-1329A Printed in U.S.A. June, 1996

0V

Hold Integrate Hold Reset

Positive or Negative

Signal Integration

S

1

S

2

IIN(t)

V

O

=

–1

∫

dt

C

INT

2

®

IVC102

The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes
no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to change
without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant
any BURR-BROWN product for use in life support devices and/or systems.

NOTES: (1) Standard test timing: 1ms integration, 200µs hold, 100µs reset. (2) Hold mode output voltage after 1ms integration of zero input current. Includes op
amp offset voltage, integration of input error current and switch charge injection effects.

SPECIFICATIONS

At TA = +25°C, VS = ±15V, RL = 2kΩ, C

INT

= C1 + C2 + C3, 1ms integration period

(1)

, unless otherwise specified.

IVC102P, U

PARAMETER CONDITIONS MIN TYP MAX UNITS
TRANSFER FUNCTION V

O

= –(IIN)(T

INT

)/C

INT

Gain Error C

INT

= C1 + C2 + C

3

±5 +25/–17 %

vs Temperature ±25 ppm/°C

Nonlinearity V

O

= ±10V ±0.005 %
Input Current Range ±100 µA
Offset Voltage

(2)

IIN = 0, CIN = 50pF –5 ±20 mV
vs Temperature ±30 µV/°C
vs Power Supply V

S

= +4.75/–10 to +18/–18V 150 750 µV/V

Droop Rate, Hold Mode –1 nV/µs

OP AMP

Input Bias Current S

1

, S2 Open –100 ±750 fA

vs Temperature See Typical Curve

Offset Voltage (Op Amp V

OS

) ±0.5 ±5mV
vs Temperature ±5 µV/°C
vs Power Supply V

S

= +4.75/–10 to +18/–18V 10 100 µV/V

Noise Voltage f = 1kHz 10 nV/√Hz

INTEGRATION CAPACITORS

C

1

+ C2 + C

3

80 100 120 pF

vs Temperature ±25 ppm/°C

C

1

10 pF

C

2

30 pF

C

3

60 pF

OUTPUT

Voltage Range, Positive R

L

= 2kΩ (V+)–3 (V+)–1.3 V

Negative R

L

= 2kΩ (V–)+3 (V–)+2.6 V
Short-Circuit Current ±20 mA
Capacitive Load Drive 500 pF
Noise Voltage See Typical Curve

DYNAMIC CHARACTERISTIC

Op Amp Gain-Bandwidth 2 MHz
Op Amp Slew Rate 3V/µs
Reset

Slew Rate 3V/µs
Settling Time, 0.01% 10V Step 6 µs

DIGITAL INPUTS (TTL/CMOS Compatible)
V

IH

(referred to digital ground) (Logic High) 2 5.5 V

V

IL

(referred to digital ground) (Logic Low) –0.5 0.8 V

I

IH

VIH = 5V 2 µA

I

IL

VIL = 0V 0 µA

Switching Time 100 ns

POWER SUPPLY

Voltage Range: Positive +4.75 +15 +18 V

Negative –10 –15 –18 V

Current: Positive 4.1 5.5 mA

Negative –1.6 –2.2 mA
Analog Ground –0.2 mA
Digital Ground –2.3 mA

TEMPERATURE RANGE

Operating Range –40 85 °C
Storage –55 125 °C
Thermal Resistance,

θ

JA

DIP 100 °C/W
SO-14 150 °C/W

3

®

IVC102

ELECTROSTATIC
DISCHARGE SENSITIVITY

This integrated circuit can be damaged by ESD. Burr-Brown
recommends that all integrated circuits be handled with
appropriate precautions. Failure to observe proper handling
and installation procedures can cause damage.

ESD damage can range from subtle performance degrada­tion to complete device failure. Precision integrated circuits
may be more susceptible to damage because very small
parametric changes could cause the device not to meet its
published specifications.

ABSOLUTE MAXIMUM RATINGS

Supply Voltage, V+ to V– .................................................................... 36V

Logic Input Voltage ...................................................................... V– to V+

Output Short Circuit to Ground ............................................... Continuous

Operating Temperature ................................................. –40°C to +125°C

Storage Temperature..................................................... –55°C to +125°C

Lead Temperature (soldering, 10s) ................................................. 300°C

PIN CONNECTIONS

Top View 14-Pin DIP/

SO-14 Surface Mount

PACKAGE INFORMATION

PACKAGE DRAWING

PRODUCT PACKAGE NUMBER

(1)

IVC102P 14-Pin DIP 010
IVC102U SO-14 Surface Mount 235

NOTE: (1) For detailed drawing and dimension table, please see end of data
sheet, or Appendix D of Burr-Brown IC Data Book.

V+
Digital Ground
S

2



S

1



V

O

V–



NC

Analog Ground

I

IN



–In

C

1



C

2



C

3 

NC

NC = No Internal Connection
Connect to Analog Ground for Lowest Noise

14
13
12
11
10

9
8

1
2
3
4
5
6
7

4

®

IVC102

10 100 1000

C

IN

(pF)

TOTAL OUTPUT NOISE vs C

IN

1000

100

10

1

Noise Voltage (µVrms)

rms Variation
of 100 Measurement
Cycles, T

INT

= 1ms.

C

INT

= 10pF

C

INT

= 30pF

C

INT

= 100pF

C

INT

= 300pF

C

INT

= 1000pF

Reset Mode, S1 Open, S2 Closed.

TYPICAL PERFORMANCE CURVES

At TA = +25°C, VS = ±15V, RL = 2kΩ, C

INT

= C1 + C2 + C3, 1ms integration period, unless otherwise specified.

–50 –25 0 25 50 75 100 125

Temperature (°C)

INPUT BIAS CURRENT vs TEMPERATURE

100p

10p

1p

100f

10f

Input Bias Current (A)

S1, S2 Open

0 100 200 300 400 500 600 700 800 900 1000

C

INT

(pF)

RESET TIME vs C

INT

30

25

20

15

10

5

0



Reset Time (µs)

Time Required to
Reset from ±10V
to 0V.

0.01%

1%

10 100 1000

Input Capacitance, C

IN

(pF)

S

1

CHARGE INJECTION vs INPUT CAPACITANCE

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2
0

Charge Injection, ∆Q (pC)

100pF

 ∆V

O

=

∆Q

100pF

S

1

C

IN

10 100 1000

Input Capacitance, C

IN

(pF)

S

2

CHARGE INJECTION vs INPUT CAPACITANCE

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1
0

Charge Injection, ∆Q (pC)

(V+) = +18V

(V+) = +15V

(V+) = +4.75V

100pF

 ∆V

O

=

∆Q

100pF

C

IN

S

2

5

®

IVC102

Charge Injection

of S

2

Op Amp V

OS

+

I

IN

• R

S2

0V

Integrate

(S

2

Open)

T

1

0V

S

2

V

O

T

2

10µs

Reset

10µs

Reset

I

IN

Photodiode

60pF

30pF

10pF

0.1µF

0.1µF

1

2

3

4

5

6

11 12 13

10 V

O

14

V+

+15V

Logic

High

(+5V)

S

1

C

1

C

2

C

3

S

2

Digital

Ground

Analog

Ground

S

2

9

–15V

V–

Digital
Data

Sampling

A/D

Converter

See timing

signal below

APPLICATION INFORMATION

Figure 1 shows the basic circuit connections to operate the
IVC102. Bypass capacitors are shown connected to the
power supply pins. Noisy power supplies should be avoided
or decoupled and carefully bypassed.

The Analog Ground terminal, pin 1, is shown internally
connected to the non-inverting input of the op amp. This
terminal connects to other internal circuitry and should be
connected to ground. Approximately 200µA flows out of
this terminal.

Digital Ground, pin 13, should be at the same voltage
potential as analog ground (within 100mV). Analog and
Digital grounds should be connected at some point in the
system, usually at the power supply connections to the
circuit board. A separate Digital Ground is provided so that
noisy logic signals can be referenced to separate circuit
board traces.

Integrator capacitors C

1

, C2 and C3 are shown connected in

parallel for a total C

INT

= 100pF. The IVC102 can be used
for a wide variety of integrating current measurements. The
input signal connections and control timing and C

INT

value
will depend on the sensor or signal type and other applica­tion details.

BASIC RESET-AND-INTEGRATE MEASUREMENT

Figure 1 shows the circuit and timing for a simple reset-and­integrate measurement. The input current is connected di­rectly to the inverting input of the IVC102, pin 3. Input
current is shown flowing out of pin 3, which produces a
positive-going ramp at V

O

. Current flowing into pin 3 would

produce a negative-going ramp.
A measurement cycle starts by resetting the integrator output

voltage to 0V by closing S

2

for 10µs. Integration of the input

current begins when S

2

opens and the input current begins to

charge C

INT

. VO is measured with a sampling a/d converter
at the end of an integration period, just prior to the next reset
period. The ideal result is proportional to the average input
current (or total accumulated charge).

Switch S

2

is again closed to reset the integrator output to 0V

before the next integration period.
This simple measurement arrangement is suited to many

applications. There are, however, limitations to this basic
approach. Input current continues to flow through S

2

during

the reset period. This leaves a small voltage on C

INT

equal

to the input current times R

S2

, the on-resistance of S2,

approximately 1.5kΩ.

FIGURE 1. Reset-and Integrate Connections and Timing.

Figure 1b

Figure 1a

6

®

IVC102

C

INT

for constant IIN, at the end of T

INT

VO = –I

IN

T

INT

C

INT

V

O

I

IN

IIN(t)

V

O

=

–1

∫

dt

C

INT

In addition, the offset voltage of the internal op amp and
charge injection of S

2

contribute to the voltage on C

INT

at the

start of integration.
Performance of this basic approach can be improved by

sampling V

O

after the reset period at T1 and subtracting this

measurement from the final sample at T

2

. Op amp offset

voltage, charge injection effects and I•R

S2

offset voltage on

S

2

are removed with this two-point measurement. The effec­tive integration period is the time between the two measure­ments, T

2-T1

.

COMPARISON TO CONVENTIONAL TRANSIMPEDANCE AMPLIFIERS

With the conventional transimpedance amplifier circuit
of Figure 2a, input current flows through the feedback
resistor, R

F

, to create a proportional output voltage.

V

O

= –IIN R

F

The transimpedance gain is determined by RF. Very large
values of R

F

are required to measure very small signal

current. Feedback resistor values exceeding 100MΩ are
common.

The IVC102 (Figure 2b) provides a similar function,
converting an input current to an output voltage. The
input current flows through the feedback capacitor, C

INT

,
charging it at a rate that is proportional to the input
current. With a constant input current, the IVC102’s
output voltage is

V

O

= –IIN T

INT/CINT

after an integration time of T

INT

.

V

O

is proportional to the integration time, T

INT

, and

inversely proportional to the feedback capacitor, C

INT

.

The effective transimpedance gain is T

INT/CINT

. Ex­tremely high gain that would be impractical to achieve
with a conventional transimpedance amplifier can be
achieved with small integration capacitor values and/or
long integration times. For example the IVC102 with
C

INT

= 100pF and T

INT

= 100ms provides an effective

transimpedance of 1GΩ. A 10nA input current would
produce a 10V output after 100ms integration.

The integrating behavior of the IVC102 reduces noise by
averaging the input noise of the sensor, amplifier, and
external sources.

Conventional Transimpedance Amplifier

Figure 2a

Integrating Transimpedance Amplifier

Figure 2b

R

F

VO = –IIN R

F

V

O

I

IN

CURRENT-OUTPUT SENSORS

Figure 3 shows a model for many current-output sensors
such as photodiodes and ionization chambers. Sensor output
is a signal-dependent current with a very high source resis­tance. The output is generally loaded into a low impedance

FIGURE 2. Comparison to a Conventional Transimpedance Amplifier.

so that the terminal voltage is kept very low. Typical sensor
capacitance values range from 10pF to over 100pF. This
capacitance plays a key role in operation of the switched­input measurement technique (see next section).

Provides time-continuous output
voltage proportional to I

IN

.

Output voltage after integration period is
proportional to average I

IN

throughout

the period.

7

®

IVC102

60pF

30pF

10pF

0.1µF

0.1µF

1

I

Photodiode
Sensor

RC

2

3

4

5

6

11

I: Signal - Dependent Current
R: Sensor Resistance
C: Sensor Capacitance

12 13

10 V

O

14

V+

+15V

S

1

S

1

C

1

C

2

C

3

S

2

S

2

9

–15V

V–

Digital
Data

A/D

Converter

See timing

signals below

3a

3b

3c

Charge transferred
from sensor C
to C

INT

.

A

A

B

B

Transfer Function

Offset Voltage

Ramp due to

input bias current

(exaggerated).

Effective

Signal Integration

Period, T

S

VO waveform with
approx. half-scale input current.

V

O

waveform with

zero input current.

∆Q

S

1

Opening

0V

0V

0V

V

O

S

2

S

1

V

O

0V

+10mV

–10mV

10µs
Hold

10µs

Reset

10µs

Hold

10µs

Reset

10µs

Pre-Int.

Hold

∆Q

S

2

Opening

∆Q

S

1

Closing

Op Amp

V

OS

(S1 Open) (S1 Closed)

(S

2

Open)

FIGURE 3. Switched-Input Measurement Technique.

Input connections and timing are shown in Figure 3.
The timing diagram, Figure 3b, shows that S

1

is closed only

when S

2

is open. During the short period that S1 is open

(30µs in this timing example), any signal current produced
by the sensor will charge the sensor’s source capacitance.
This charge is then transferred to C

INT

when S1 is closed. As
a result, no charge produced by the sensor is lost and the
input signal is continuously integrated. Even fast input
pulses are accurately integrated.

SWITCHED-INPUT MEASUREMENT TECHNIQUE

While the basic reset-and-integrate measurement arrange­ment in Figure 1 is satisfactory for many applications, the
switched-input timing technique shown in Figure 3 has
important advantages. This method can provide continuous
integration of the input signal. Furthermore, it can hold the
output voltage constant after integration for stable conver­sion (desirable for a/d converter without a sample/hold).

8

®

IVC102

The input current, IIN, is shown as a conventional current
flowing into pin 2 in this diagram but the input current could
be bipolar (positive or negative). Current flowing out of pin
2 would produce a positive-ramping V

O

.

The timing sequence proceeds as follows:

Reset Period

The integrator is reset by closing switch S

2

with S1 open. A

10µs reset time is recommended to allow the op amp to slew
to 0V and settle to its final value.

Pre-Integration Hold

S

2

is opened, holding VO constant for 10µs prior to integra-

tion. This pre-integration hold period assures that S

2

is fully

open before S

1

is closed so that no input signal is lost. A

minimum of 1µs is recommended to avoid switching over­lap. The 10µs hold period shown in Figure 3b also allows an
a/d converter measurement to be made at point A. The
purpose of this measurement at A is discussed in the “Offset
Errors” section.

Integration on C

INT

Integration of the input current on C

INT

begins when S1 is
closed. An immediate step output voltage change occurs as
the charge that was stored on the input sensor capacitance is
transferred to C

INT

. Although this period of charging C

INT

occurs only while S1 is closed, the charge transferred as S

1

is closed causes the effective integration time to be equal to
the complete conversion period—see Figure 3b.

The integration period could range from 100µs to many
minutes, depending on the input current and C

INT

value.

While S

1

is closed, IIN charges C

INT

, producing a negative-

going ramp at the integrator output voltage, V

O

. The output
voltage at the end of integration is proportional to the
average input current throughout the complete conversion
cycle, including the integration period, reset and both hold
periods.

Hold Period

Opening S

1

halts integration on C

INT

. Approximately 5µs

after S

1

is opened, the output voltage is stable and can be

measured (at point B). The hold period is 10µs in this
example. C

INT

remains charged until a S2 is again closed, to

reset for the next conversion cycle.
In this timing example, S

1

is open for a total of 30µs. During

this time, signal current from the sensor charges the sensor
source capacitance. Care should be used to assure that the
voltage developed on the sensor does not exceed approxi­mately 200mV during this time. The I

IN

terminal, pin 2, is
internally clamped with diodes. If these diodes forward bias,
signal current will flow to ground and will not be accurately
integrated.

A maximum of 333nA signal current could be accurately
integrated on a 50pF sensor capacitance for 30µs before
200mV would be developed on the sensor.

I

MAX

= (50pF) (200mV) / 30µs = 333nA

OFFSET ERRORS

Figure 3c shows the effect on V

O

due to op amp input offset
voltage, input bias current and switch charge injection. It
assumes zero input current from the sensor. The various
offsets and charge injection (∆Q) jumps shown are typical of
that seen with a 50pF source capacitance. The specified
“transfer function offset voltage” is the voltage measured
during the hold period at B. Transfer function offset voltage
is dominated by the charge injection of S

2

opening and op

amp V

OS

. The opening and closing charge injections of S

1

are very nearly equal and opposite and are not significant
contributors.

Note that using a two-point difference measurement at A
and B can dramatically reduce offset due to op amp V

OS

and

S

2

charge injection. The remaining offset with this B-A
measurement is due to op amp input bias current charging
C

INT

. This error is usually very small and is exaggerated in

the figure.

DIGITAL SWITCH INPUTS

The digital control inputs to S

1

and S2 are compatible with
standard CMOS or TTL logic. Logic input pins 11 and 12
are high impedance and the threshold is approximately 1.4V
relative to Digital Ground, pin 13. A logic “low” closes the
switch.

Use care in routing these logic signals to their respective
input pins. Capacitive coupling of logic transitions to sensi­tive input nodes (pins 2 through 6) and to the positive power
supply (pin 14) will dramatically increase charge injection
and produce errors. Route these circuit board traces over a
ground plane (digital ground) and route digital ground traces
between logic traces and other critical traces for lowest
charge injection. See Figure 4.

5V logic levels are generally satisfactory. Lower voltage
logic levels may help reduce charge injection errors, de­pending on circuit layout. Logic high voltages greater than

5.5V, or higher than the V+ supply are not recommended.

FIGURE 4. Circuit Board Layout Techniques.

•

••

•

•

•

•

•

•

•

•

•

•

•

•

Input trace guarded
all the way to sensor.

Switch logic inputs
guarded by digital
ground.

Analog

Ground

Digital

Ground

Pins 7 and 8 have no internal 

connection but are connected to

ground for lowest noise pickup.

V+

Input nodes
guarded by
analog ground.

V–

V

O

S

1

S

2

14

8

7

1

9

®

IVC102

CHOOSING C

INT

Internal capacitors C1, C2 and C3 are high quality metal/
oxide types with low leakage and excellent dielectric char­acteristics. Temperature stability is excellent—see typical
curve. They can be connected for C

INT

= 10pF, 30pF, 40pF,

60pF, 70pF, 90pF or 100pF. Connect unused internal ca­pacitor pins to analog ground. Accuracy is ±20%, which
directly influences the gain of the transfer function.

A larger value external C

INT

can be connected between pins
3 and 10 for slower/longer integration. Select a capacitor
type with low leakage and good temperature stability.
Teflon



, polystyrene or polypropylene capacitors generally
provide excellent leakage, temperature drift and voltage
coefficient characteristics. Lower cost types such as NPO
ceramic, mica or glass may be adequate for many applica­tions. Larger values for C

INT

require a longer reset time—see

typical curves.

FREQUENCY RESPONSE

Integration of the input signal for a fixed period produces a
deep null (zero response) at the frequency 1/T

INT

and its
harmonics. An ac input current at this frequency (or its
harmonics) has zero average value and therefore produces
no output. This property can be used to position response
nulls at critical frequencies. For example, a 16.67ms integra­tion period produces response nulls at 60Hz, 120Hz, 180Hz,
etc., which will reject ac line frequency noise and its har­monics. Response nulls can be positioned to reduce interfer­ence from system clocks or other periodic noise.
Response to all frequencies above f = 1/T

INT

falls at –20dB/
decade. The effective corner frequency of this single-pole
response is approximately 1/2.8T

INT

.

For the simple reset-and-integrate measurement technique,
T

INT

is equal to the to the time that S2 is open. The switched­input technique, however, effectively integrates the input
signal throughout the full measurement cycle, including the
reset period and both hold periods. Using the timing shown
in Figure 3, the effective integration time is 1/Ts, where Ts
is the repetition rate of the sampling.

INPUT IMPEDANCE

The input impedance of a perfect transimpedance circuit is
zero ohms. The input voltage ideally would be zero for any
input current. The actual input voltage when directly driving
the integrator input (pin 3) is proportional to the output slew
rate of the integrator. A 1V/µs slew rate produces approxi­mately 100mV at pin 3. The input of the integrator can be
modeled as a resistance:

R

IN

= 10–7/C

INT

with RIN in Ω and C

INT

in Farads.

Using the internal C

INT

= C1 + C2 + C3 = 100pF

R

IN

= 10–7/ 100pF = 1kΩ

(2)

(3)

INPUT BIAS CURRENT ERRORS

Careful circuit board layout and assembly techniques are
required to achieve the very low input bias current capability
of the IVC102. The critical input connections are at ground
potential, so analog ground should be used as a circuit board
guard trace surrounding all critical nodes. These include
pins 2, 3, 4, 5 and 6. See Figure 4.

Input bias current increases with temperature—see typical
performance curve Input Bias Current vs Temperature.

HOLD MODE DROOP

Hold-mode droop is a slow change in output voltage prima­rily due to op amp input bias current. Droop is specified
using the internal C

INT

= 100pF and is based on a –100fA
typical input bias current. Current flows out of the inverting
input of the internal op amp.

With C

INT

= 100pF, the droop rate is typically only

1nV/µs—slow enough that it rarely contributes significant
error at moderate temperatures.

Since the input bias current increases with temperature, the
droop rate will also increase with temperature. The droop
rate will approximately double for each 10°C increase in
junction temperature—see typical curves.

Droop rate is inversely proportional to C

INT

. If an external
integrator capacitor is used, a low leakage capacitor should
be selected to preserve the low droop performance of the
IVC102.

INPUT CURRENT RANGE

Extremely low input currents can be measured by integrat­ing for long periods and/or using a small value for C

INT

.
Input bias current of the internal op amp is the primary
source of error.

Larger input currents can be measured by increasing the
value of C

INT

and/or using a shorter integration time. Input

currents greater than 200µA should not be applied to the pin
2 input, however. The approximately 1.5kΩ series resistance
of S

1

will create an input voltage at pin 2 that will begin to
forward-bias internal protection clamp diodes. Any current
that flows through these protection diodes will not be accu­rately integrated. See “Input Impedance” section for more
information on input current-induced voltage.

Input current greater than 200µA can, however, be con­nected directly to pin 3, using the simple reset-integrate
technique shown in Figure 1. Current applied at this input
can be externally switched to avoid excessive I•R voltage
across S

2

during reset. Inputs up to 5mA at pin 3 can be

accurately integrated if C

INT

is made large enough to limit

slew rate to less than 1V/µs. A 5mA input current would
require C

INT

= 5nF to produce a 1V/µs slew rate. The input

current appears as load current to the internal op amp,
reducing its ability to drive an external load.

Droop Rate =

–100fA

C

INT

Teflon E. I. Du Pont de Nemours & Co.

10

®

IVC102

The input resistance seen at pin 2 includes an additional

1.5kΩ, the on-resistance of S

1

. The total input resistance is

the sum of the switch resistance and R

IN

, or 2.5kΩ in this

example.

Slew rate limit of the internal op amp is approximately
3V/µs. For most applications, the slew rate of V

OUT

should

be limited to 1V/µs or less. The rate of change is propor­tional to I

IN

and inversely proportional to C

INT

:

This can be important in some applications since the slew­induced input voltage is applied to the sensor or signal
source. The slew-induced input voltage can be reduced by
increasing C

INT

, which reduces the output slew rate.

NONLINEARITY

Careful nonlinearity measurements of the IVC102 yield
typical results of approximately ±0.005% using the internal
input capacitors (C

INT

= 100pF). Nonlinearity will be de­graded by using an external integrator capacitor with poor
voltage coefficient. Performance with the internal capacitors
is typically equal or better than the sensors it is used to
measure. Actual application circuits with sensors such as a
photodiode may have other sources of nonlinearity.

1/10T

INT

1/T

INT

10/T

INT

●●●

–20dB/decade

slope

Frequency

0

–10

–20

–30

–40

–50

Frequency Response (dB)

Corner at

f = 0.32/T

INT

–3dB at

f = 0.44/T

INT

FIGURE 5. Frequency Response of Integrating Converter.

Slew Rate =

I

IN

C

INT