LTC6242IGN datasheet(16/24 Pages) LINER

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LTC6241/LTC6242

62412f

APPLICATIO S I FOR ATIO

phase lead to compensate the phase lag of the second

ampliﬁer. The op amp’s good input offset voltage match

and low input bias current means that the typical differential

output voltage is less than 40µV. A noise spectrum plot of

the differential output is shown in Figure 5.

gain of the difference ampliﬁer is one. An LTC6910-2 PGA

ampliﬁes the difference ampliﬁer output with inverting

gains of –1, –2, –4, –8, –16, –32 and –64. The second

LTC6241 op amp is used as an integrator to set the DC

output voltage equal to the LT6650 reference voltage VREF.

The integrator drives the PGA analog ground to provide

a feedback loop, in addition to blocking any DC voltage

through the PGA. The reference voltage of the LT6650

can be set to a voltage from 400mV to V+ – 350mV with

resistors R5 and R6. If R6 is 20k or less, the error due

to the LT6650 op amp bias current is negligible. The low

voltage offset and drift of the LTC6241 integrator will not

contribute any signiﬁcant error to the LT6650 reference

voltage. The LT6650 VREF voltage has a maximum error

Figure 5. Differential Output Noise

FREQUENCY (kHz)

020

90 100

140

100

120

6241 F05

VS = ±2.5V

TA = 25°C

–3dB BW = 80kHz

Achieving Low Input Bias Current

The DD package is leadless and makes contact to the PCB

beneath the package. Solder ﬂux used during the attach-

ment of the part to the PCB can create leakage current

paths and can degrade the input bias current performance

of the part. All inputs are susceptible because the backside

paddle is connected to V– internally. As the input voltage

changes or if V– changes, a leakage path can be formed

and alter the observed input bias current. For lowest bias

current, use the LTC6241 in the SO-8 and provide a guard

ring around the inputs that are tied to a potential near the

input voltage.

A Digitally Programmable AC Difference Ampliﬁer

The LTC6241 conﬁgured as a difference ampliﬁer, can

be combined with a programmable gain ampliﬁer (PGA)

to obtain a low noise high speed programmable differ-

ence ampliﬁer. Figure 6 shows the LTC6241 based as a

single-supply AC ampliﬁer. One LTC6241 op amp is used

at the circuit’s input as a standard four resistor difference

ampliﬁer. The low bias current and current noise of the

LTC6241 allow the use of high valued input resistors, 100k

or greater. Resistors R1, R2, R3 and R4 are equal and the

Figure 6. Wideband Difference Ampliﬁer with High

Input Impedance and Digitally Programmable Gain

6241 F06

–

1/2

LTC6241

µF

0.1

µF

765

234

AGND

OUT

V–

V+

0.1

µF

V+

R1 = R2 = R3 = R4

1000pF

20k

LTC6910-2

LT6650

VOUT

VREF

1/2

LTC6241

100

Ω

µF

DIGITAL INPUTS

GAIN

–1

–2

–4

–8

–16

–32

–64

VOUT = (V1 – V2) GAIN + VREF

()

REF

⎛

⎝⎜

⎞

⎠⎟

()

510

R6 20

.•

••

–

d BANDWIDTH

HIGH

LOW

HIGH

––

••

•

GAIN

LOW =

••

•

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