INA337, INA338

10

SBOS222A

www.ti.com

mum possible input stage gain for best accuracy and lowest

noise.

Circuit layout and supply bypassing can affect performance.

Minimize the stray capacitance on pins 1 and 8. Use recom-

mended supply bypassing, including a capacitor directly from

pin 7 to pin 4 (V+ to V–), even with dual (split) power supplies

(see Figure 1).

DYNAMIC PERFORMANCE

The typical characteristic “Gain vs Frequency” shows that the

INA337 has nearly constant bandwidth regardless of gain.

This results from the bandwidth limiting from the recom-

mended filters.

NOISE PERFORMANCE

Internal auto-correction circuitry eliminates virtually all 1/f

noise (noise that increases at low frequency) in gains of 100

or greater. Noise performance is affected by gain-setting

resistor values. Follow recommendations in the “Setting

Gain” section for best performance.

Total noise is a combination of input stage noise and output

stage noise. When referred to the input, the total mid-band

noise is:

VnV

Hz

nV

Hz

G

33

800

/

/

(3)

The output noise has some 1/f components that affect

performance in gains less than 10. See typical characteristic

“Input-Referred Voltage Noise vs Frequency.”

High-frequency noise is created by internal auto-correction

circuitry and is highly dependent on the filter characteristics

chosen. This may be the dominant source of noise visible

when viewing the output on an oscilloscope. Low cutoff

frequency filters will provide lowest noise. Figure 2 shows the

typical noise performance as a function of cutoff frequency.

Applications sensitive to the spectral characteristics of high-

frequency noise may require consideration of the spurious

frequencies generated by internal clocking circuitry. “Spurs”

occur at approximately 90kHz and its harmonics (see typical

characteristic “Input Referred Ripple”) which may be reduced

by additional filtering below 1kHz.

Insufficient filtering at pin 5 can cause nonlinearity with large

output voltage swings (very near the supply rails). Noise

must be sufficiently filtered at pin 5 so that noise peaks do not

“hit the rail” and change the average value of the signal.

Figure 2 shows guidelines for filter cutoff frequency.

HIGH-FREQUENCY NOISE

correction circuitry noise. Filter frequencies can be chosen to

optimize the tradeoff between noise and frequency response

of the application, as shown in Figure 2. The cutoff frequen-

cies of the filters are generally set to the same frequency.

Figure 2 shows the typical output noise for four gains as a

function of the –3dB cutoff frequency of each filter response.

Small signals may exhibit the addition of internally generated

auto-correction circuitry noise at the output. This noise,

combined with broadband noise, becomes most evident in

higher gains with filters of wider bandwidth.

INPUT BIAS CURRENT RETURN PATH

The input impedance of the INA337 is extremely high—

Ω. However, a path must be provided for

the input bias current of both inputs. This input bias current is

approximately

±0.2nA. High input impedance means that this

input bias current changes very little with varying input voltage.

Input circuitry must provide a path for this input bias current

for proper operation. Figure 3 shows provisions for an input

bias current path in a thermocouple application. Without a

bias current path, the inputs will float to an undefined poten-

tial and the output voltage may not be valid.

FIGURE 2. Total Output Noise vs Filter Cutoff Frequency.

100

1

10

1k

10k

Required Filter Cutoff Frequency (Hz)

1k

100

10

1

G = 10

G = 1

G = 100

G = 1000

INA337

Thermocouple

5

FIGURE 3. Providing Input Bias Current Return Path.