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LTC6404

6404f

APPLICATIONS INFORMATION

Functional Description

The LTC6404 is a small outline, wide band, low noise,

and low distortion fully-differential ampliﬁer with accurate

output phase balancing. The LTC6404 is optimized to drive

low voltage, single-supply, differential input 14-bit to 18-bit

analog-to-digital converters (ADCs). The LTC6404’s output

is capable of swinging rail-to-rail on supplies as low as

2.7V, which makes the ampliﬁer ideal for converting ground

referenced, single-ended signals into DC level-shifted

differential signals in preparation for driving low voltage,

single-supply, differential input ADCs. Unlike traditional

op amps which have a single output, the LTC6404 has

two outputs to process signals differentially. This allows

for two times the signal swing in low voltage systems

when compared to single-ended output ampliﬁers. The

balanced differential nature of the ampliﬁer also provides

even-order harmonic distortion cancellation, and less

susceptibility to common mode noise (e.g., power supply

noise). The LTC6404 can be used as a single-ended input

to differential output ampliﬁer, or as a differential input to

differential output ampliﬁer.

The LTC6404’s output common mode voltage, deﬁned

as the average of the two output voltages, is independent

of the input common mode voltage, and is adjusted by

applying a voltage on the VOCM pin. If the pin is left open,

there is an internal resistive voltage divider that develops

a potential halfway between the V+ and V– pins. Whenever

this pin is not hard tied to a low impedance ground plane,

it is recommended that a high quality ceramic capacitor is

used to bypass the VOCM pin to a low impedance ground

plane (See Layout Considerations in this document). The

LTC6404’s internal common mode feedback path forces

accurate output phase balancing to reduce even order

harmonics, and centers each individual output about the

potential set by the VOCM pin.

OUTCM

OCM

OUT

–

The outputs (OUT+ and OUT–) of the LTC6404 are capable

of swinging rail-to-rail. They can source or sink up to ap-

proximately 65mA of current.

Additional outputs (OUTF+ and OUTF–) are available that

provide ﬁltered versions of the OUT+ and OUT– outputs. An

on-chip single pole RC passive ﬁlter band limits the ﬁltered

outputs to a –3dB frequency of 88.5MHz. The user has a

choice of using the unﬁltered outputs, the ﬁltered outputs,

or modifying the ﬁltered outputs to adjust the frequency

response by adding additional components.

In applications where the full bandwidth of the LTC6404 is

desired, the unﬁltered outputs (OUT+ and OUT–) should be

used. The unﬁltered outputs OUT+ and OUT– are designed

to drive 10pF to ground (or 5pF differentially). Capacitances

greater than 10pF will produce excess peaking, and can

be mitigated by placing at least 25

Ω in series with each

output pin.

Input Pin Protection

The LTC6404’s input stage is protected against differential

input voltages which exceed 1.4V by two pairs of back-

to-back diodes connected in anti-parallel series between

IN+ and IN– (Pins 6 and 15). In addition, the input pins

have steering diodes to either power supply. If the input

pair is overdriven, the current should be limited to under

10mA to prevent damage to the IC. The LTC6404 also has

steering diodes to either power supply on the VOCM and

SHDN pins (Pins 4 and 1), and if forced to voltages which

exceed either supply, they too, should be current-limited

to under 10mA.

SHDN Pin

If the SHDN pin (Pin 1) is pulled 2.1V below the posi-

tive supply, circuitry is activated which powers down

the LTC6404. The pin will have the Thevenin equivalent

impedance of approximately 66k

Ω to V+. If the pin is left

unconnected, an internal pull-up resistor of 150k will

keep the part in normal active operation. Care should

be taken to control leakage currents at this pin to under

1μA to prevent inadvertently putting the LTC6404 into

shutdown. In shutdown, all biasing current sources are

shut off, and the output pins, OUT+ and OUT–, will each

appear as open collectors with a non-linear capacitor in

parallel and steering diodes to either supply. Because of

the non-linear capacitance, the outputs still have the ability

to sink and source small amounts of transient current if

driven by signiﬁcant voltage transients. The inputs (IN+,

and IN–) appear as anti-parallel diodes which can conduct

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