Datasheet

OPA693
20
SBOS285A
www.ti.com
The exceptionally linear output stage (as illustrated by the
high 3rd-order intermodulation intercept) and low thermal
gradient induced errors for the OPA693 give an extremely
linear output over large voltage swings and heavy loads.
Figure 13 shows the tested deviation (in % of peak to peak)
from linearity for a range of symmetrical output swings and
loads. Below 4V
PP
, for either a 100Ω or a 500Ω load, the
OPA693 delivers > 14-bit linear output response.
The total output spot noise voltage can be computed as the
square root of the sum of all squared output noise voltage
contributors. Equation 1 shows the general form for the
output noise voltage using the terms shown in Figure 14.
(1)
E E I R kTR NG I R kTR NG
O
NI BN
SS
BI F F
=+
( )
+




+
( )
+
2
2
2
2
44
Dividing this expression through by noise gain (NG = 1 + R
F
/R
G
)
will give the equivalent input-referred spot noise voltage at the
non-inverting input, as shown in Equation 2.
(2)
E E I R kTR
IR
NG
kTR
NG
NNIBN
SS
BI F F
=+
( )
++






+
2
2
2
4
4
Evaluating the output noise and input noise expressions for
the two noninverting gain configurations, and with two differ-
ent values for the noninverting source impedance, gives
output and input referred spot noise voltages of Table II.
OUTPUT TOTAL INPUT
SPOT NOISE SPOT NOISE
R
S
E
O
E
N
CONFIGURATION (Ω)(nV/
√Hz
)(nV/
√Hz
)
G = +2 (Figure 1) 25 8.3 4.15
G = +2 (Figure 1) 300 14 7
G = +1 (Figure 2) 25 7.3 7.3
G = +1 (Figure 2) 300 9.2 9.2
TABLE II. Total Output and Input Referred Noise.
4kT
R
G
R
G
R
F
R
S
OPA693
I
BI
E
O
I
BN
4kT = 1.6E –20J
at 290°K
E
RS
E
NI
√4kTR
S
√4kTR
F
Figure 14. Op Amp Noise Model.
The output noise is being dominated by the inverting current
noise times the internal feedback resistor. This gives a total
input referred noise voltage that exceeds the 1.8nV voltage
term for the amplifier itself.
DC ACCURACY AND OFFSET CONTROL
A current-feedback op amp like the OPA693 provides excep-
tional bandwidth and slew rate giving fast pulse settling but
only moderate DC accuracy. The Electrical Characteristics
show an input offset voltage comparable to high-speed volt-
age-feedback amplifiers. However, the two input bias currents
are somewhat higher and are unmatched. Whereas bias
current cancellation techniques are very effective with most
voltage-feedback op amps, they do not generally reduce the
output DC offset for wideband current-feedback op amps.
Since the two input bias currents are unrelated in both mag-
nitude and polarity, matching the source impedance looking
out of each input to reduce their error contribution to the output
is ineffective. Evaluating the configuration of Figure 1, using
worst case +25°C input offset voltage and the two input bias
currents, gives a worst-case output offset range equal to:
±(NG × V
OS
) + (I
BN
× R
S
/2 × NG) ± (I
BI
× R
F
)
= ±(2 × 2.0mV) ± (35µA × 25Ω × 2) ± (50µA × 300Ω)
= ±4mV ± 1.75mV ± 15mV
= ±30.75mV
where NG = noninverting signal gain.
Figure 13. DC Linearity vs Output Swing and Loads.
0.0200
0.0175
0.0150
0.0125
0.0100
0.0075
0.0050
0.0025
0
V
O
(peak to peak)
234 5678
% Deviation
Figure 1 Test Circuit
R
L
= 100Ω
R
L
= 500Ω
NOISE PERFORMANCE
The OPA693 offers an excellent balance between voltage and
current noise terms to achieve a low output noise under a
variety of operating conditions. The inverting node noise
current (internal) will appear at the output multiplied by the
relatively low 300Ω feedback resistor. The input noise voltage
(1.8nV/
√Hz ) is extremely low for a unity gain stable amplifier.
This low input voltage noise was achieved at the price of
higher noninverting input current noise (17.8pA/
√Hz ). As long
as the AC source impedance looking out of the noninverting
input is less than 100Ω, this current noise will not contribute
significantly to the total output noise. The op amp input voltage
noise and the two input current noise terms combine to give
low output noise for the each of the three gain settings
available using the OPA693. Figure 14 shows the op amp
noise analysis model with all of the noise terms included. In
this model, all noise terms are taken to be noise voltage or
current density terms in either nV/
√Hz or pA/ √Hz .