OPA121KU Equivalent & Substitute Parts

Part Overview

The OPA121KU is a general-purpose operational amplifier manufactured by Texas Instruments, housed in an 8-SOIC surface-mount package. This device features a Difet® input stage and is classified as obsolete, with 1602 pieces currently in stock. The OPA121KU operates across a wide supply voltage range (10V to 36V) and is suitable for applications requiring low input bias current and moderate bandwidth performance.

Due to its obsolete status, identifying equivalent and substitute parts is necessary for design continuity, long-term availability assurance, and potential performance optimization in new designs. Substitute parts must maintain compatibility with the 8-SOIC package format and support the required supply voltage range while meeting or exceeding key electrical specifications.

Key Parameters

Parameter	OPA121KU Value	Unit
Amplifier Type	General Purpose	—
Number of Circuits	1	—
Package / Case	8-SOIC (0.154", 3.90mm Width)	—
Slew Rate	2	V/µs
Gain Bandwidth Product	2	MHz
Current - Input Bias	1	pA
Voltage - Input Offset	500	µV
Current - Supply	2.5	mA
Current - Output / Channel	10	mA
Voltage - Supply Span (Min)	10	V
Voltage - Supply Span (Max)	36	V
Operating Temperature	0°C ~ 70°C	—
Mounting Type	Surface Mount	—
Product Status	Obsolete	—
RoHS Status	ROHS3 Compliant	—
Moisture Sensitivity Level (MSL)	3 (168 Hours)	—

Substitute Part Grouping Explanation

Substitution of the OPA121KU is determined by strict adherence to the following electrical and mechanical parameters:

Primary Compatibility Criteria:

Package format: 8-SOIC (0.154", 3.90mm Width) surface-mount configuration
Supply voltage range: Minimum 10V to maximum 36V overlap required
Single-circuit operational amplifier topology
Surface-mount mounting type

Secondary Electrical Criteria:

Slew rate: 2V/µs or greater
Gain bandwidth product: 2 MHz or greater
Input bias current: 1 pA or lower
Input offset voltage: 500 µV or lower
Supply current: 2.5 mA or lower
Output current per channel: 10 mA or greater

The substitute parts are grouped into two categories based on amplifier type and performance characteristics:

Category 1 – General Purpose Rail-to-Rail (TLV171IDR): Active product with enhanced temperature range and lower supply voltage minimum, suitable for modern low-power applications.

Category 2 – J-FET Input Amplifiers (AD825ARZ, AD825ARZ-REEL, AD825ARZ-REEL7, AD8510BRZ, AD8510BRZ-REEL, AD8510BRZ-REEL7): Active products offering superior slew rate and bandwidth performance, with extended temperature operating ranges and improved input offset voltage specifications.

Parameter Comparison

Parameter	OPA121KU	TLV171IDR	AD825ARZ	AD825ARZ-REEL	AD825ARZ-REEL7	AD8510BRZ	AD8510BRZ-REEL	AD8510BRZ-REEL7
Amplifier Type	General Purpose	General Purpose	J-FET	J-FET	J-FET	J-FET	J-FET	J-FET
Number of Circuits	1	1	1	1	1	1	1	1
Package / Case	8-SOIC	8-SOIC	8-SOIC	8-SOIC	8-SOIC	8-SOIC	8-SOIC	8-SOIC
Slew Rate (V/µs)	2	1.5	140	140	140	20	20	20
Gain Bandwidth Product (MHz)	2	3	26	26	26	8	8	8
Current - Input Bias (pA)	1	10	15	15	15	25	25	25
Voltage - Input Offset (µV)	500	750	1000	1000	1000	80	80	80
Current - Supply (mA)	2.5	0.525	6.5	6.5	6.5	2.2	2.2	2.2
Current - Output / Channel (mA)	10	—	50	50	50	70	70	70
Voltage - Supply Span Min (V)	10	2.7	10	10	10	9	9	9
Voltage - Supply Span Max (V)	36	36	36	36	36	36	36	36
Operating Temperature (°C)	0 ~ 70	-40 ~ 125	-40 ~ 85	-40 ~ 85	-40 ~ 85	-40 ~ 125	-40 ~ 125	-40 ~ 125
Mounting Type	Surface Mount	Surface Mount	Surface Mount	Surface Mount	Surface Mount	Surface Mount	Surface Mount	Surface Mount
Product Status	Obsolete	Active	Active	Active	Active	Active	Active	Active
RoHS Status	ROHS3 Compliant	ROHS3 Compliant	ROHS3 Compliant	ROHS3 Compliant	ROHS3 Compliant	ROHS3 Compliant	ROHS3 Compliant	ROHS3 Compliant
MSL Rating	3 (168 Hours)	2 (1 Year)	1 (Unlimited)	1 (Unlimited)	1 (Unlimited)	1 (Unlimited)	1 (Unlimited)	1 (Unlimited)

Engineering Selection Recommendations

For Direct Replacement (Minimum Specification Match):

The TLV171IDR is the primary recommended substitute for the OPA121KU. This Texas Instruments general-purpose amplifier maintains the same manufacturer lineage and package format. The TLV171IDR exceeds the OPA121KU in gain bandwidth product (3 MHz vs. 2 MHz) and offers superior supply current efficiency (525 µA vs. 2.5 mA). The extended operating temperature range (-40°C to 125°C) and improved moisture sensitivity level (MSL 2 vs. MSL 3) provide enhanced reliability. The TLV171IDR is an active product with full RoHS3 compliance and REACH unaffected status.

For Performance-Enhanced Applications:

The AD8510BRZ, AD8510BRZ-REEL, and AD8510BRZ-REEL7 (all variants of the AD8510 base product) provide superior slew rate (20 V/µs vs. 2 V/µs) and gain bandwidth product (8 MHz vs. 2 MHz) while maintaining comparable supply current (2.2 mA vs. 2.5 mA). These J-FET input amplifiers deliver significantly improved input offset voltage (80 µV vs. 500 µV) and support extended operating temperature ranges (-40°C to 125°C). All variants are active products with MSL 1 (Unlimited) rating and full compliance certifications.

For High-Speed Applications:

The AD825ARZ, AD825ARZ-REEL, and AD825ARZ-REEL7 (all variants of the AD825 base product) deliver exceptional slew rate (140 V/µs) and gain bandwidth product (26 MHz), suitable for applications requiring high-frequency performance. These J-FET amplifiers provide 50 mA output current per channel and operate across the -40°C to 85°C temperature range. All variants maintain MSL 1 (Unlimited) rating and full compliance certifications.

Packaging Considerations:

All substitute parts are available in 8-SOIC surface-mount package format, ensuring direct PCB footprint compatibility. Packaging variants (Tube, Cut Tape & Digi-Reel®) affect supply chain logistics but do not alter electrical or mechanical specifications. The AD825ARZ-REEL7 and AD8510BRZ-REEL7 variants offer the highest inventory availability (18,103 and 1,940 pieces respectively).

Compliance and Regulatory Status:

All substitute parts maintain ROHS3 compliance and REACH unaffected status, matching the OPA121KU regulatory profile. The transition from obsolete to active product status ensures long-term availability and manufacturing continuity.

Frequently Asked Questions (FAQ)

Q1: Can the TLV171IDR directly replace the OPA121KU without circuit modifications?

The TLV171IDR is pin-compatible and functionally equivalent within the 8-SOIC package. Both devices are single-circuit operational amplifiers with identical pinout. The TLV171IDR exceeds OPA121KU specifications in gain bandwidth product and slew rate while consuming less supply current. No circuit modifications are required for direct substitution in applications operating within the OPA121KU's original temperature range (0°C to 70°C). Applications utilizing the extended temperature capability of the TLV171IDR (-40°C to 125°C) may require thermal design review.

Q2: What is the primary difference between the AD825 and AD8510 J-FET amplifier families?

Both families are J-FET input operational amplifiers in 8-SOIC packages with supply voltage ranges compatible with the OPA121KU (10V to 36V maximum). The AD825 series delivers superior slew rate (140 V/µs vs. 20 V/µs) and gain bandwidth product (26 MHz vs. 8 MHz), making it suitable for high-frequency signal processing. The AD8510 series provides lower input offset voltage (80 µV vs. 1 mV) and comparable supply current to the OPA121KU (2.2 mA), making it suitable for precision measurement applications. Selection depends on whether the application prioritizes bandwidth or offset voltage performance.

Q3: Are the AD825ARZ, AD825ARZ-REEL, and AD825ARZ-REEL7 electrically identical?

Yes. All three variants (AD825ARZ, AD825ARZ-REEL, AD825ARZ-REEL7) are electrically and functionally identical. The designations reflect packaging and supply chain format only: AD825ARZ is supplied in Tube packaging, while AD825ARZ-REEL and AD825ARZ-REEL7 are supplied in Cut Tape & Digi-Reel® format. Electrical specifications, performance characteristics, and compliance certifications are identical across all variants. Selection among variants is determined by procurement and assembly process requirements.

Q4: What does MSL (Moisture Sensitivity Level) rating mean for component substitution?

Moisture Sensitivity Level indicates the maximum time a component can be exposed to ambient conditions (typically 30°C, 85% relative humidity) before requiring baking prior to soldering. The OPA121KU has MSL 3 (168 hours), while all substitute parts have MSL 1 or MSL 2 ratings. Lower MSL numbers indicate superior moisture resistance and longer shelf life without baking requirements. This represents an improvement in handling and storage convenience for all substitute parts.

Q5: Can I use the AD8510BRZ-REEL7 in place of the OPA121KU in a legacy design?

The AD8510BRZ-REEL7 is pin-compatible and functionally superior to the OPA121KU within the 8-SOIC package. The AD8510BRZ-REEL7 provides 10× lower input offset voltage (80 µV vs. 500 µV), 10× higher slew rate (20 V/µs vs. 2 V/µs), and 4× higher gain bandwidth product (8 MHz vs. 2 MHz). Supply voltage range compatibility is maintained (9V to 36V minimum vs. 10V to 36V for OPA121KU). The extended operating temperature range (-40°C to 125°C) exceeds the OPA121KU specification. Direct substitution is supported; no circuit modifications are required.

Q6: Which substitute part offers the best supply current efficiency?

The TLV171IDR offers the lowest supply current at 525 µA, representing a 79% reduction compared to the OPA121KU (2.5 mA). This makes the TLV171IDR the optimal choice for battery-powered or low-power applications. The AD8510 family (AD8510BRZ, AD8510BRZ-REEL, AD8510BRZ-REEL7) provides the next best efficiency at 2.2 mA, matching the OPA121KU supply current specification. The AD825 family (AD825ARZ, AD825ARZ-REEL, AD825ARZ-REEL7) requires higher supply current (6.5 mA) due to enhanced bandwidth performance.

Q7: Are all substitute parts available in the same 8-SOIC package footprint?

Yes. All substitute parts are housed in 8-SOIC (0.154", 3.90mm Width) surface-mount packages, ensuring identical PCB footprint compatibility with the OPA121KU. No PCB redesign or layout modifications are required for mechanical integration. Packaging variants (Tube, Cut Tape & Digi-Reel®) affect component supply format and handling but do not alter the physical package dimensions or pinout.

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