LT1793AIN8#PBF Datasheet LT1793ACN8#PBF, LT1793IS8#PBF, LT1793AIS8#PBF | P7-P9

LT1793

CCHARA TERIST

ICS

LPER

Short-Circuit Output Current

vs Temperature

TEMPERATURE (°C)

–75

OUTPUT CURRENT (mA)

–25 5025

100

1793 G19

–50 0

125

SINK SOURCE

= ±15V

Supply Current vs Temperature

TEMPERATURE (°C)

–75

SUPPLY CURRENT PER AMPLIFIER (mA)

–25 5025

100

1793 G20

–50 0

125

= ±15V

= ±5V

I FOR ATIO

LT1793 vs the Competition

With improved noise performance, the LT1793 in the

PDIP directly replaces such JFET op amps as the OPA111

and the AD645. The combination of low current and

voltage noise of the LT1793 allows it to surpass most dual

and single JFET op amps. The LT1793 can replace many

of the lowest noise bipolar amps that are used in amplify-

ing low level signals from high impedance transducers.

The best bipolar op amps (with higher current noise) will

eventually lose out to the LT1793 when transducer im-

pedance increases.

Figure 1. Comparison of LT1793, OP215, and AD822

Input Bias Current vs Common Mode Range

COMMON MODE RANGE (V)

–15

–100

INPUT BIAS CURRENT (pA)

–60

–40

–20

–10

–5

1793 F01

100

–80

LT1793

AD822

CURRENT NOISE = √2qI

OP215

TEMPERATURE (°C)

INPUT BIAS AND OFFSET CURRENTS (A)

300p

100p

30n

10n

100

1793 G21

30p

10p

0.3p

125

= ±15V

= –10 TO 13V

BIAS

CURRENT

OFFSET

CURRENT

Input Bias and Offset Currents

vs Chip Temperature

The extremely high input impedance (10

Ω) assures that

the input bias current is almost constant over the entire

common mode range. Figure 1 shows how the LT1793

stands up to the competition. Unlike the competition, as the

input voltage is swept across the entire common mode

range the input bias current of the LT1793 hardly changes.

As a result the current noise does not degrade. This makes

the LT1793 the best choice in applications where an

amplifier has to buffer signals from a high impedance

transducer.

Offset nulling will be compatible with these devices with the

wiper of the potentiometer tied to the negative supply

(Figure 2a). No appreciable change in offset voltage drift

∆V

= ±13mV

50k

15V

–15V

–

∆V

= ±1.3mV

50k

10k

15V

–15V

(b)(a)

1793 F02

–

Figure 2

LT1793

I FOR ATIO

voltage noise, the thermal noise of the transducer, and the

op amp’s input bias current noise times the transducer

impedance.

Figure 3 shows total input voltage noise

versus source resistance. In a low source resistance

(<5k) application the op amp voltage noise will dominate

the total noise. This means the LT1793 is superior to

most JFET op amps. Only the lowest noise bipolar op

amps have the advantage at low source resistances. As

the source resistance increases from 5k to 50k, the

LT1793 will match the best bipolar op amps for noise

perfor

mance, since the thermal noise of the transducer

(4kTR) begins to dominate the total noise. A further

increase in source resistance, above 50k, is where the op

amp’s current noise component (2qI

) will eventually

dominate the total noise. At these high source resis-

tances, the LT1793 will out perform the lowest noise

bipolar op amps due to the inherently low current noise of

FET input op amps. Clearly, the LT1793 will extend the

range of high impedance transducers that can be used for

high signal-to-noise ratios. This makes the LT1793 the

best choice for high impedance, capacitive transducers.

Optimization Techniques for Charge Amplifiers

The high input impedance JFET front end makes the

LT1793 suitable in applications where very high charge

sensitivity is required. Figure 4 illustrates the LT1793 in its

inverting and noninverting modes of operation. A charge

amplifier is shown in the inverting mode example; the gain

depends on the principal of charge conservation at the

input of the LT1793. The charge across the transducer

capacitance C

is transferred to the feedback capacitor C

with temperature will occur when the device is nulled with

a potentiometer ranging from 10k to 200k. Finer adjust-

ments can be made with resistors in series with the

potentiometer (Figure 2b).

Amplifying Signals from High Impedance Transducers

The low voltage and current noise offered by the LT1793

makes it useful in a wide range of applications, especially

where high impedance, capacitive transducers are used

such as hydrophones, precision accelerometers and

photodiodes. The total output noise in such a system is

the gain times the RMS sum of the op amp’s input referred

Figure 3. Comparison of LT1793 and LT1007 Total Output

1kHz Voltage Noise vs Source Resistance

Figure 4. Inverting and Noninverting Gain Configurations

SOURCE RESISTANCE (Ω)

100

10k

1k 100M 1G

1793 F03

100k

100

10M10k 1M

INPUT NOISE VOLTAGE (nV/√Hz)

= A

√V

(OP AMP)

+ 4kTR

+ 2

SOURCE RESISTANCE = 2R

= R

* PLUS RESISTOR

†

PLUS RESISTOR  1000pF CAPACITOR

RESISTOR NOISE ONLY

LT1793

LT1007*

LT1007

†

LT1793

†

LT1007

LT1793*

–

OUTPUT

≅ C

= R

> R1

OR R2

TRANSDUCER

–

OUTPUT

= C

C

= R

R

TRANSDUCER

1793 F04

Q = CV; = I = C

LT1793

I FOR ATIO

Input: ±5.2V Sine Wave

LT1793 Output

LT1793 F05a LT1793 F05b

Figure 5. Voltage Follower with Input Exceeding the Common Mode Range (V

= ±5V)

resulting in a change in voltage dV, which is equal to dQ/C

The gain therefore is C

. For unity-gain, the C

should

equal the transducer capacitance plus the input capaci-

tance of the LT1793 and R

should equal R

In the noninverting mode example, the transducer current

is converted to a change in voltage by the transducer

capacitance, C

. This voltage is then buffered by the

LT1793 with a gain of 1 + R1/R2. A DC path is provided by

, which is either the transducer impedance or an exter-

nal resistor. Since R

is usually several orders of magni-

tude greater than the parallel combination of R1 and R2, R

is added to balance the DC offset caused by the noninvert-

ing input bias current and R

. The input bias currents,

although small at room temperature, can create significant

errors at higher temperature, especially with transducer

resistances of up to 1000M or more. The optimum value

for R

is determined by equating the thermal noise (4kTR

)

to the current noise (2qI

) times R

. Solving for R

results in R

= R

= 2V

= 26mV at 25°C). A parallel

capacitor C

, is used to cancel the phase shift caused by

the op amp input capacitance and R

Reduced Power Supply Operation

To take full advantage of a wide input common mode range,

the LT1793 was designed to eliminate phase reversal.

Referring to the photographs in Figure 5, the LT1793 is

shown operating in the follower mode (A

= 1) at ±5V

supplies with the input swinging ±5.2V. The output of the

LT1793 clips cleanly and recovers with no phase reversal.

This has the benefit of preventing lockup in servo systems

and minimizing distortion components.

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LT1793AIN8#PBF

Mfr. #:

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Manufacturer:

Analog Devices Inc.

Description:

Operational Amplifiers - Op Amps L N, pAere Bias C, JFET In Op Amp

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LT1793AIN8#PBF

LT1793AIN8#PBF

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